Two cards are face up, one card is face down.
After the equals sign, a card shows how much all the cards make together.
Add up the face-up cards and fill in the rest up to the total.
Example: 20 and 30 are face up, all cards together make 75.
20 + 30 = 50, and from 50 up to 75 there are 25 missing.
So the face-down card is 25.
Later a minus takes the place of the plus, for example 72 − ? = 45.
Then you look for the number that was taken away: from 45 up to 72 it is 27.
The number filled in shows the solution.
Example
For parents
What does this exercise practice?
Work out a missing number by filling up to a result (the reverse of adding, later also of subtracting).
Common pitfalls
Some children add all three numbers. A helpful question is: how far is it from the face-up cards to the result? With minus, the face-down card is the distance from the result up to the face-up card.
How to practice at home
Practice with playing cards, turn one card over and say the total.
Related: Inverse problems, What does multiplying mean?