arithmetic - if 20 men can build a water fountain 56 metres long in 6 days what length of a similar water fountain can be built by 35 men in 3 days - skillgun

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 6: 3 : : 56 : x
Therefore (20 x 6 x x)=(35 x 3 x 56)
x=(35 x 3 x 56)/120=49
Hence, the required length is 49 m.

what if there is indirect proportion how to solve it for exam if 18 men can build a wall 140 meters long in 42 days, in how many days would 15 men will build a wall 100 meter long?