At 7 am, Ben and his sister left their house and cycled to school. Ben cycled at a speed of 4km/h faster than his sister. When Ben reached his school, he realised he had left his wallet at home and he immediately cycled back home to get his wallet. After cycling 400m from school, he met his sister on the way.

a. What time did Ben meet his sister on his way back?

b. What was the distance from Ben’s house to the school?