400
You're late to school, which starts at 8am. You usually walk to school, which takes you 20 minutes. Today you're late so you're riding your bike, which takes you 1/4 of the time it takes to walk. Based on this you expect to be 2 minutes late to school. What time did you leave the house?
7:57
(If it takes 1/4 the time to walk to school, then that means it takes 5 minutes. Since you're two minutes late for school--and it takes a total of 5 minutes--you must have left at 7:57)