If 20 men can build a water fountain 56 metres long in 42 days, what length of a similar water fountain can be built by 35 men in 3 days?  Pick one:  A) 3 m  B) 4 m  C) 7 m  D) 9 m  E) 10 m

Rationale: "explanation : let the required length be x metres more men , more length built ( direct proportion ) less days , less length built ( direct proportion ) men 20 : 35 days 42 : 3 : : 56 : x therefore ( 20 x 42 x x ) = ( 35 x 3 x 56 ) x = ( 35 x 3 x 56 ) / 840 = 7 hence , the required length is 7 m . answer : c"
Answer: C