Answer: 3/4 of the original price.
Step-by-step explanation:
Ok, so Sam buys 4 boxes in Store A.
We know that in Store A you can get 4 boxes paying only for 3.
If P is the price of a Box, then he got 4 boxes and paid 3*P
then the amount that he paid for each box is equal to the total amount that he paid divided the number of boxes that he received.
this is: 3P/4 = (3/4)*P
Meaning that he paid 3/4 of the original price for each box.