Answer:
This problem has two possible solutions.
2232
7236
Step-by-step explanation:
To be divisible by 72, a number has:
To be divisible by both 8 and 9 at the same time.
It is divisible by 8 if it's last two digits are divisible by 4.
It is divisible by 9 it the sum of it's digits is a number divisible by 9.
a23b
3b must be divisible by 4. In the thirties, the numbers that are divisible by 4 are 32 and 36. So b = 2 or b = 6.
a232
2 + 3 + 2 = 7
The higest possible value of a is 9 and the lowest is 1.
Between 8 and 16, only 9 is divisible by 9. So a = 2.
a = 2, b = 2 is one of the solutions.
a236
2 + 3 + 6 = 11
The higest possible value of a is 9 and the lowest is 1.
Between 12 and 20, only 18 is divisible by 9. So we need a = 7
ft tall and the finished stack of bricks is 
ft tall.