diff --git a/demo.txt b/demo.txt
new file mode 100644
index 00000000..ce013625
--- /dev/null
+++ b/demo.txt
@@ -0,0 +1 @@
+hello
diff --git a/package.json b/package.json
index 04d6b866..7838202a 100644
--- a/package.json
+++ b/package.json
@@ -3,6 +3,7 @@
     "test": "jasmine-node --verbose specs"
   },
   "dependencies": {
-    "jasmine-node": "1.14.5"
+    "jasmine-node": "1.14.5",
+    "lodash": "^4.14.0"
   }
-}
\ No newline at end of file
+}
diff --git a/practices/superposition_operation/collection_operation/letter_mapping_1.js b/practices/superposition_operation/collection_operation/letter_mapping_1.js
index 2a2dba26..3f5138d8 100644
--- a/practices/superposition_operation/collection_operation/letter_mapping_1.js
+++ b/practices/superposition_operation/collection_operation/letter_mapping_1.js
@@ -1,8 +1,14 @@
 'use strict';
 
-function even_to_letter(collection) {
+let _ = require('lodash');
 
-  //在这里写入代码
+function even_to_letter(collection) {
+  let diff = 'a'.charCodeAt(0) - 1;
+  return _(collection)
+    .filter(x => x%2===0)
+    .map(x => x + diff)
+    .map(x => String.fromCharCode(x))
+    .value();
 }
 
 module.exports = even_to_letter;
diff --git a/practices/superposition_operation/collection_operation/letter_mapping_2.js b/practices/superposition_operation/collection_operation/letter_mapping_2.js
index 20970ced..19eccddc 100644
--- a/practices/superposition_operation/collection_operation/letter_mapping_2.js
+++ b/practices/superposition_operation/collection_operation/letter_mapping_2.js
@@ -1,8 +1,14 @@
 'use strict';
 
-function average_to_letter(collection) {
+let _ = require('lodash');
 
-  //在这里写入代码
+function average_to_letter(collection) {
+  let diff = 'a'.charCodeAt(0) - 1;
+  let average = _.chain(collection)
+    .mean()
+    .ceil()
+    .value();
+  return String.fromCharCode(average + diff);
 }
 
 module.exports = average_to_letter;
diff --git a/practices/superposition_operation/collection_operation/practice_1.js b/practices/superposition_operation/collection_operation/practice_1.js
index 6ecb445a..a390a29e 100644
--- a/practices/superposition_operation/collection_operation/practice_1.js
+++ b/practices/superposition_operation/collection_operation/practice_1.js
@@ -1,8 +1,9 @@
 'use strict';
 
-function hybrid_operation(collection) {
+let _ = require('lodash');
 
-  //在这里写入代码
+function hybrid_operation(collection) {
+    return _(collection).map(x => x*3+2).sum();
 }
 
 module.exports = hybrid_operation;
diff --git a/practices/superposition_operation/collection_operation/practice_2.js b/practices/superposition_operation/collection_operation/practice_2.js
index f8e2dd6e..21dbe213 100644
--- a/practices/superposition_operation/collection_operation/practice_2.js
+++ b/practices/superposition_operation/collection_operation/practice_2.js
@@ -1,8 +1,12 @@
 'use strict';
 
-function hybrid_operation_to_uneven(collection) {
+let _ = require('lodash');
 
-  //在这里写入代码
+function hybrid_operation_to_uneven(collection) {
+  return _(collection)
+    .filter(x => x%2==1)
+    .map(x => x*3+2)
+    .value();
 }
 
 module.exports = hybrid_operation_to_uneven;
diff --git a/practices/superposition_operation/collection_operation/practice_3.js b/practices/superposition_operation/collection_operation/practice_3.js
index f8e2dd6e..31f31536 100644
--- a/practices/superposition_operation/collection_operation/practice_3.js
+++ b/practices/superposition_operation/collection_operation/practice_3.js
@@ -1,7 +1,12 @@
 'use strict';
 
-function hybrid_operation_to_uneven(collection) {
+let _ = require('lodash');
 
+function hybrid_operation_to_uneven(collection) {
+ return _(collection)
+   .filter(x => x%2===1)
+   .map(x => x*3+5)
+   .sum();
   //在这里写入代码
 }
 
diff --git a/practices/superposition_operation/interval_operation/amount_even.js b/practices/superposition_operation/interval_operation/amount_even.js
index 9f35f5a4..ae5be8d7 100644
--- a/practices/superposition_operation/interval_operation/amount_even.js
+++ b/practices/superposition_operation/interval_operation/amount_even.js
@@ -1,7 +1,11 @@
 'use strict';
 
-function amount_even(collection) {
+let _ = require('lodash');
 
+function amount_even(collection) {
+ return _(collection)
+   .filter(x => x%2===0)
+   .sum();
   //在这里写入代码
 }
 
diff --git a/practices/superposition_operation/interval_operation/average_uneven.js b/practices/superposition_operation/interval_operation/average_uneven.js
index 48cacdd9..a19f6917 100644
--- a/practices/superposition_operation/interval_operation/average_uneven.js
+++ b/practices/superposition_operation/interval_operation/average_uneven.js
@@ -1,8 +1,11 @@
 'use strict';
 
-function average_uneven(collection) {
+let _ = require('lodash');
 
-  //在这里写入代码
+function average_uneven(collection) {
+  return _(collection)
+    .filter(x=>x%2===1)
+    .mean();
 }
 
 module.exports = average_uneven;
diff --git a/practices/superposition_operation/interval_operation/median_to_letter.js b/practices/superposition_operation/interval_operation/median_to_letter.js
index 922d214a..428d7ef3 100644
--- a/practices/superposition_operation/interval_operation/median_to_letter.js
+++ b/practices/superposition_operation/interval_operation/median_to_letter.js
@@ -1,8 +1,24 @@
 'use strict';
 
-function median_to_letter(collection) {
+let _ = require('lodash');
+
+function _toCharNumber(charCode) {
+  // TODO
+  return 0;
+}
 
-  //在这里写入代码
+function median_to_letter(collection) {
+  let median = _.chain(collection)
+    .sortBy()
+    .filter((item, index, all) => {
+      if(all.length%2===1) {
+        return index === (all.length-1)/2;
+      } else {
+        let expected = [all.length/2, all.length/2+1]
+        return expected.includes(index);
+      }
+    });
+  return _toCharNumber(median);
 }
 
 module.exports = median_to_letter;
diff --git a/specs/superposition_operation/collection_operation/letter_mapping_1_spec.js b/specs/superposition_operation/collection_operation/letter_mapping_1_spec.js
index d662bd9f..1fa2ddeb 100644
--- a/specs/superposition_operation/collection_operation/letter_mapping_1_spec.js
+++ b/specs/superposition_operation/collection_operation/letter_mapping_1_spec.js
@@ -2,7 +2,7 @@
 
 var even_to_letter = require("../../../practices/superposition_operation/collection_operation/letter_mapping_1.js");
 
-describe('even_to_letter', function() {
+ddescribe('even_to_letter', function() {
 
   var collection = [1,2,3,4,5,6,7,8,9,10];
 
diff --git a/specs/superposition_operation/collection_operation/letter_mapping_2_spec.js b/specs/superposition_operation/collection_operation/letter_mapping_2_spec.js
index 2cc0eb58..2044be67 100644
--- a/specs/superposition_operation/collection_operation/letter_mapping_2_spec.js
+++ b/specs/superposition_operation/collection_operation/letter_mapping_2_spec.js
@@ -2,7 +2,7 @@
 
 var average_to_letter = require("../../../practices/superposition_operation/collection_operation/letter_mapping_2.js");
 
-describe('average_to_letter', function() {
+ddescribe('average_to_letter', function() {
 
   var collection = [1,2,3,4,5,6,7,8,9,10];
 
diff --git a/specs/superposition_operation/collection_operation/practice_1_spec.js b/specs/superposition_operation/collection_operation/practice_1_spec.js
index f59196e4..43b8b9b5 100644
--- a/specs/superposition_operation/collection_operation/practice_1_spec.js
+++ b/specs/superposition_operation/collection_operation/practice_1_spec.js
@@ -2,7 +2,7 @@
 
 var hybrid_operation = require("../../../practices/superposition_operation/collection_operation/practice_1.js");
 
-describe('hybrid_operation', function() {
+ddescribe('hybrid_operation', function() {
 
   var collection = [1,5,7,11,35,67];
 
diff --git a/specs/superposition_operation/collection_operation/practice_2_spec.js b/specs/superposition_operation/collection_operation/practice_2_spec.js
index ccbd8ae3..18f20712 100644
--- a/specs/superposition_operation/collection_operation/practice_2_spec.js
+++ b/specs/superposition_operation/collection_operation/practice_2_spec.js
@@ -2,7 +2,7 @@
 
 var hybrid_operation_to_uneven = require("../../../practices/superposition_operation/collection_operation/practice_2.js");
 
-describe('hybrid_operation_to_uneven', function() {
+ddescribe('hybrid_operation_to_uneven', function() {
 
   var collection = [1,5,7,12,11,35,54,67,70];
 
diff --git a/specs/superposition_operation/collection_operation/practice_3_spec.js b/specs/superposition_operation/collection_operation/practice_3_spec.js
index a9e91a26..4056ff09 100644
--- a/specs/superposition_operation/collection_operation/practice_3_spec.js
+++ b/specs/superposition_operation/collection_operation/practice_3_spec.js
@@ -2,7 +2,7 @@
 
 var hybrid_operation_to_uneven = require("../../../practices/superposition_operation/collection_operation/practice_3.js");
 
-describe('hybrid_operation_to_uneven', function() {
+ddescribe('hybrid_operation_to_uneven', function() {
 
   var collection = [1,5,7,12,11,35,54,67,70];
 
diff --git a/specs/superposition_operation/interval_operation/amount_even_spec.js b/specs/superposition_operation/interval_operation/amount_even_spec.js
index aa15c48c..b772a536 100644
--- a/specs/superposition_operation/interval_operation/amount_even_spec.js
+++ b/specs/superposition_operation/interval_operation/amount_even_spec.js
@@ -2,7 +2,7 @@
 
 var amount_even = require("../../../practices/superposition_operation/interval_operation/amount_even.js");
 
-describe('amount_even', function() {
+ddescribe('amount_even', function() {
 
   var collection = [1,2,3,4,5,6,7,8,9,10];
 
diff --git a/specs/superposition_operation/interval_operation/average_uneven_spec.js b/specs/superposition_operation/interval_operation/average_uneven_spec.js
index 3db01a78..9330e059 100644
--- a/specs/superposition_operation/interval_operation/average_uneven_spec.js
+++ b/specs/superposition_operation/interval_operation/average_uneven_spec.js
@@ -2,7 +2,7 @@
 
 var average_uneven = require("../../../practices/superposition_operation/interval_operation/average_uneven.js");
 
-describe('average_uneven', function() {
+ddescribe('average_uneven', function() {
 
   var collection = [1,2,3,4,5,6,7,8,9,10];
 
diff --git a/specs/superposition_operation/interval_operation/median_to_letter_spec.js b/specs/superposition_operation/interval_operation/median_to_letter_spec.js
index b58fffaf..673546ab 100644
--- a/specs/superposition_operation/interval_operation/median_to_letter_spec.js
+++ b/specs/superposition_operation/interval_operation/median_to_letter_spec.js
@@ -1,8 +1,8 @@
 'use strict';
 
-var median_to_letter = require("../../../practices/superposition_operation/interval_operation/average_uneven.js");
+var median_to_letter = require("../../../practices/superposition_operation/interval_operation/median_to_letter.js");
 
-describe('median_to_letter', function() {
+ddescribe('median_to_letter', function() {
 
   var collection = [20,21,22,23,24,25,26,27,28,29,
                     30,31,32,33,34,35,36,37,38,39,