In a grass field, If there are 40 cows, they could eat for
40 days. If there are 30 cows, they could eat for 60 days.
Than if 20 cows, How much day they could eat?
a) 60 b) 80 c)55 d)50

Answer Posted / subash

All, the choice given are wrong ...

Let 'x' be the amount of grazed by a cow per day,

Let 'y' be the growth rate of grass per day,

Let 'z' be the amount of grass present initially,

So, 40 cows can graze (40*x) amount of grass per day and
(40*x*40) amount of grass for 40 days.

For 40 days, the growth of grass is (40*y)

So, (40*x*40) = (z + 40*y)

For 30 cows it takes 60 days, so,

(30*x*60) = (z + 60*y)

so, 1600x - 40y - z = 0 ----- 1

1800x - 60y - z = 0 ----- 2

Subtracting we get, 10x = y , Sub. 10x=y in 1 or 2, we get z
= 120y or z = 1200x

For, 20 cows,

20*x*N = z + N*y

Sub, 10x=y and (z=120y or 1200x), we get N = 120

Therefore, for 20 cows it takes 120 days to graze.

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