Use | S − S N | ≤ b N + 1 to find the smallest value of N such that S N approximates the value of the sum S to within an error of at most 10 − 4 . S = [infinity] ∑ n = 1 ( − 1 ) n + 1 n ( n + 4 ) ( n + 5 )

Sam opened a money-market account that pays 3% simple interest. he started the account with $7,000 and made no further deposits. when he closed the account, he had earned $420 in interest. how long did he keep his account open?