If 30 men can build a wall 56 meters long in 5 days, what length of a similar wall can be built by 40 men in 3 days ?

Let the required length be x meters
More men, More length built (Direct proportion)
Less days, Less length built (Direct Proportion)
(30 x 5 x X) = (40 x 3 x 56)
=> x = 44.8
Hence, the required length is 44.8 m.