Aclass is collecting quarters for a charity drive. after 3 weeks, they have collected 120 quarters. after another week, the total number of quarters is at 210. how many more quarters were collected during the fourth week?

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?