add context to within 10% excercise

[fherreazcue]

maybe add a tip about the mathematical questions that lead to "within 10%", so something like:

Remember that x is only within 10% of y if:   0.9 < x/y < 1.1

Some student's have no idea of how to start...

[fherreazcue]

this came up again today!

[fherreazcue]

The solution with

0.9<=a/b<=1.1

has problems with b==0, but the alternative:

a>=0.9b && a<=1.1b

does not work for negative numbers...
Catching the case b==0 is probably easier.