Ask someone to pick any number between 1 and 31.
Then, ask them to point to each card on which their number appears.
You can tell them their number by simply adding up the numbers
at the top left-hand corner of all the cards that they pointed to.
For Example :
Say someone picks the number 13.
They would then point to the green (1), the blue (4), and the yellow cards (8).
Simply sum 1 + 4 + 8 = 13, and tell them the number they picked was 13!