There are even numbers of similar number hidden under the black box. The player needs to click to see the number of the box. Next, if the player clicks on another box containing same number as previous box then the second box will be opened. If the second box the user clicked on contain different number, then the first opened box will be closed.
Thus by clicking, viewing, and memorising numbers, all the box containing duplicate number will be opened.
Demo Link - https://cutt.ly/1v2dVVs