Answer: There are 1296 crayons have been used altogether.
Explanation:
Since we have given that
Number of boxes = 15
Number of crayons in each box = 64
So, Number of crayons in 15 boxes would be

Number of boxes = 12
Number of crayons in each box = 28
So, Number of crayons in 12 boxes would be

So, total number of crayons altogether would be

Hence, there are 1296 crayons have been used altogether.