I think $6.75 for 15 juice boxes is better because, each juice box is 0.45.
While $5.50 for 11 juice boxes is 0.50.
And if we were to add 0.50 enough times to make 15 juice boxes it would equal 7.50
RIP OFFFFFFF
THE SECOND ONE IS A WAY BETTER OF A DEAL
Given that 1/3 of a box is 6 dollars, we can say that per dollar is 1/18 of a box. If you have 4 dollars, this would mean that you can buy 4/18 of a box to 2/9 of a box. Hope this answers your question. Have a great day ahead!