Let's first determine how much Charlie will have in his savings account at the end of the summer if he doesn't withdraw any money. There are 12 weeks in the summer, so he would have:
$500 - $25/week x 12 weeks = $200
So if he withdraws money for more than 0 weeks, he will not have at least $200 in the account at the end of the summer. Therefore, he cannot withdraw any money from the account.