Answer:
They can buy 16 sodas.
Explanation:
We know that each soda costs $15. So, if they have $20 to buy sodas, we can find the amount of sodas by applying the rule of three:
So, if one cost $15, how many soda could they but with $20?

If we make a table would be like this:
Amount of Soda Cost ($)
1 1.25
2 2.50
3 3.75
4 5
5 6.25
6 7.50
7 8.75
8 10
9 11.25
10 12.50
11 13.75
12 15
13 16.25
14 17.50
15 18.75
16 20
Therefore, with $20 they can buy 16 sodas.