Store A is cheaper than B
The way to solve is pretty simple for B.
You have to divide the cost by the number of tubes.
So in this case:
9/3=3
15/5=3
21/7=3
See the pattern?
So the cost per lip gloss is $3.
Now for Store A:
Just divide 5.50/2
The answer is 2.75
Compare both numbers
2.75<3.00
So, store A is cheaper.