To average speed:
This problem solved by using formula "Distance = Time * Speed"
Susan can travel or cover 1 distance at 30 mph (1/30), and another distance at 60 mph (1/60). Overall she covers 2 distances, thus you should: 2 / [(1/30) + (1/60) = 2 / [3/60] = 2 * (60 / 3) = 40