Answer: The correct answer is 86.25 sit-ups (or round-up to 87 whole sit-ups).
Explanation:
S = current amount of sit-ups (75)
S2 = Increased amount of sit-ups by 15%
The equation:
S2 = S + (S x .15)
S2 = 75 + (75 x .15)
S2 = 75 + 11.25
S2 = 86.25 sit-ups