If 10 spiders can catch 10 files in 10 min, then how many flies can 200 spiders catch in 200 min?

Let the required number of flies be N.
More spiders, More flies (Direct proportion)
More time, More flies (Direct proportion)
∴ 10 x 10 x N = 200 x 200 x 10
∴ N = (200 x 200 x 10)/(10 x 10) = 4000 flies