Answer is $3.00 saved going to Gym B
Step by step
First find the rate per month for Gym A and B
A= $15 for 2 months = 15/2 = 7.50 per month
B = $21 for 3 months = 21/3 = 7.00 per month
Our equation for cost at 6 months
A = 6 x $7.50 = $45
B = 6 x $7.00 = $42
Subtract
$45 - $42 = $3.00 saved going to Gym B