Jack and Jill are marathon runners. Jack can finish a marathon (42 km) in 6 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 options:
A) 7 / 10	B) 15 / 14	C) 4 / 5	D) 5 / 4	E) can not be determined

Rationale: "average speed of jack = distance / time = 42 / 6 = 7 average speed of jill = 42 / ( 4.2 ) = 10 ratio of average speed of jack to jill = 7 / 10 answer a"
Answer: A