Mo reads a lot of books. When he was five years old he read 4 books. Each year he reads three more than twice the number of books he read the previous year.
(a) How many books does Mo read when he's six years old?
(b) How many books does Mo read when he's eight years old?
(c) Give a formula for the number of books Mo reads when he's 5 + i years old. It's ok if there's a summation sign in your formula.
(hint: When finding the answers to (a) and (b), don't simplify too much).
(d) Use your formula from (c) to calculate how many books Mo reads when he's fifteen years old.
Check your answer by computing the same number without using the formula.