Jack and Jill are marathon runners. Jack can finish a marathon (42 km) in 3.5 hours and Jill can run a marathon in 4.2 hours.What is the ratio of their average running speed? (Jack: Jill) Select.
A) 14 / 15
B) 15 / 14
C) 4 / 5
D) 6 / 5
E) can not be determined

Rationale: average speed of jack = distance / time = 42 / ( 7 / 2 ) = 84 / 7 average speed of jill = 42 / ( 4.2 ) = 10 ratio of average speed of jack to jill = ( 84 / 7 ) / 10 = 84 / 70 = 6 / 5 answer d
Answer: D