Question

The formula S=3F-24 is used to calculate shoe size,S, from food length,F(in inches).which equation could be used to calculate foot length from a given shoe size?

Answer 1

Solving
  S = 3F - 24
for F gives
  S + 24 = 3F
  (S + 24)/3 = F

The best choice is the last one:
  F = (S + 24)/3