A very easy game, but very stimulating, to motivate the mental calculation. Two students, with a number attached to each one's back, so that the rest of the class will see the numbers, but not them. We tell them the result of the sum of both numbers, and they must guess what number they are.
In the example of the photograph, the result is 10. The girl says I am 3, so I ask the boy, what numberare you, then? He will answer 7. We ask the class: are they 3 and 7. They will answer no, so they should think of another sum whose result is 10.
This game is also very good for practicing the commutative property of the sum, since the girl must guess that she is number 4 and the boy that he is number 6. For example, if they ask if they are 3 + 7, they should also check if they are 7 + 3.