If 20 men can build a water fountain 56 metres long in 14 days, what length of a similar water fountain can be built by 35 men in 3 days?  Choose the correct choice from the following options:
A) 21 m  B) 24 m  C) 27 m  D) 29 m  E) 30 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 14 : 3 : : 56 : x therefore ( 20 x 14 x x ) = ( 35 x 3 x 56 ) x = ( 35 x 3 x 56 ) / 280 = 49 hence , the required length is 21 m . answer : a"
Answer: A