If a boat is rowed downstream for 600 km in 20 hours and upstream for 48 km in 4 hours, what is the speed of the boat and the river?
Pick one:
A) 21 , 21
B) 9 , 21
C) 21 , 9
D) 9 , 9
E) 20 , 8

Rationale: "explanation : if x : speed of boats man in still water y : speed of the river downstream speed ( ds ) = x + y upstream speed ( us ) = x â € “ y x = ( ds + us ) / 2 y = ( ds â € “ us ) / 2 in the above problem ds = 30 ; us = 12 x = ( 30 + 12 ) / 2 = 42 / 2 = 21 km / hr y = ( 30 - 12 ) / 2 = 18 / 2 = 9 km / hr answer : c"
Answer: C