Let
be the amount (gal) of the 40% slime solution you end up using. You want to end up with 120 gal of the 50% solution, so that you would use
gal of the 70% solution.
You want the final solution to consist of 50% slime, or 60 gal of slime. Each gal of the 40% solution contributes 0.4 gal of slime, while each gal of the 70% solution contributes 0.7 gal of slime. This means




so the answer is B.
19,3/4 = 79/4
15,3/4 = 63/4
Perimeter = 79/4+79/4+63/4+63/4 = 284/4 = 71feet
22,1/2 = 45/2 X 4 = 90
90-71= 19 feet
Let me know if you need more of an explanation, but the answer in 19 feet (option 4)