Jack and Jill are marathon runners. Jack can finish a marathon (42 km) in 2.5 hours and Jill can run a marathon in 4.2 hours.What is the ratio of their average running speed? (Jack: Jill) Choose the correct choice from the following choices:
A) 41 / 25  B) 15 / 14  C) 4 / 5  D) 5 / 4  E) can not be determined

Rationale: "average speed of jack = distance / time = 42 / ( 5 / 2 ) = 84 / 5 average speed of jill = 42 / ( 4.2 ) = 10 ratio of average speed of jack to jill = ( 84 / 5 ) / 10 = 84 / 50 = 41 / 25 answer a"
Answer: A