Well it can be solved really easily. Firstly we will calculate for Allison. So Allison swam 21 laps in the swam athon so that means his father had to pay 3$ for every lap which means in the end he paid a total of 3 * 21 = 63.
Now as justin swam 15 laps in the swam athon so that means his father had to pay 3$ for every lap which means in the end he paid a total of 3 * 15 = 45.
So a total of 108 $ combined was paid by both of their fathers.
