Given: 
- Store A costs $6 for 3 boxes. 
 - Store B costs $5 for 2 boxes. 
 
To find: Which store has the better deal. 
Answer:
 
In order to do so, we'll need to divide the cost by the number of boxes to retrieve the cost of 1 box. The store with the lowest price for 1 box will have the better deal.
 
Let's first see what a box costs in Store A.

At Store A, the cost of a box is $2.
 
Now let's see what a box costs in Store B.

 
At Store B, the cost of a box is $2.5.
 
Since the cost at Store B seems to be a slightly higher price, Store A has the better deal.
 
Hope it helps. :)