A family daycare center charges a one-time $ 80 enrollment fee and $ 90 per week (w) Write an equation for the total cost (t) based on the number of weeks (w) that a child is enrolled.

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?