To solve this problem you need to figure out the volume of the sipping container, and the volume of the tissue box.
39*19.5*19.5=14,829.75
6.5*6.5*6.5=274.63
In order to figure out how many fits you need to divide
14,829.75/274.63=53.99
The maximum number that you can fit is the box is 53.
Hope this helps