Bob walks to Jack's house. He walks 900 meters East then realizes that he walked too far. He turns around and walks 300 meters West. The entire walk takes him 15 seconds. What was his speed per second?

Bob's average speed was 80 m/s.

His average velocity was 40 m/s East.

The magnitude of his velocity was almost 4 times as fast as Usain Bolt's world record in the 100 meters, or about 89 mph. His speed was double that.

work: w = fd (given)

work done by chet   =50*1 = 50 j

work done by mika =40*2 = 80 j

work done by sara =30*1.5 = 45 j

work done by bill =60*0.5 = 30 j

students in order from the greatest amount of work done to the least is mika> chet> sara> bill

hence option (b) is correct.