You are going to meet a friend at a skate park that is 4 miles from your house. You will ride your bike at a constant rate of 10 miles per hour. Your friend lives 1.5 miles from the skate park and will walk at a constant rate of 3 miles per hour. If each of you starts from your respective houses, how many minutes before you leave should your friend leave, so that you meet at the skate park at the same time?