Big Bob is trying to shape up for the summer. He begins on the first day of his exercise program with 3 sit ups. Each day he will do 4 more sit ups than the day before. On what day will Big Bob do 115 sit ups?

Answer 1

looks like an arithmetic sequence with terms:
3, 7, 11, ....
a = 3, d=4 , n = ? so that term(n) ≥ 115
term(n) = a + (n-1)d
6 + 4(n-1) ≥ 115
4n - 4 ≥ 109
4n ≥ 113
n ≥ 28.25

on day 28 he will do 6 + 4(27) or 114 push-ups
so he will need 29 days to reach 114 push-ups