4. The usual 110-V household alternating current varies from –155 V to 155 V with a frequency of 60 cycles/ sec ( frequency is the reciprocal of period ). Assume the current
starts at rest and then rises.

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?