Andrew works in a company that provides a generous vacation allotment: for every 10 days worked, you get 1 vacation day. If last year Andrew worked 300 days and took 5 days off in March and twice as many in September, how many more vacation days can Andrew still take? Let's be accurate as possible.
If for every 10 days worked Andrew earns 1 day of vacation, then over 300 days worked he would have earned 300 / 10 = 30 days of vacation. We know he took 5 days off in March and that in September twice as much. That means in September he took 5 * 2 = 10 days off. In total, Andrew has used 5 vacation days in March and another 10 in September. Therefore, he has used 5 + 10 = 15 vacation days. If Andrew earned 30 days of vacation but has only used 15, then he can still take another 30 - 15 = 15 days off.
The answer: 15.