Answer:
18x18 cm
Explanation:
This problem translates to the greatest common factor (GCF) of 72 and 90, which is 18.
The factors of 72 are: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72
The factors of 90 are: 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
Then the greatest common factor is 18.
4x18 = 72
5x18 = 90
so with 4x5=20 pieces of paper, the boy can be covered. (Boy?)