Answer:
21 boxes
Explanation:
To obtain the least amount of boxes Gary can put together ;
Obtain the lowest common multiple of 3 and 7
Multiples of :
3 = 3, 6, 9, 12, 15, 18, 21, 24, 27, 30,..
7 = 7, 14, 21, 28, 35, 42,...
Hence, the lowest common multiple of 3 and 7 is 21.
The least number of boxes Gary cab put together without having any cookies leftover is 21.
