Thank you D H,
I think it could be more interesting if we consider the rate of grass re-growth as per square unit. It does not seem logically correct that grass regrows irrespective of the space /area available for its growth.
Whatsoever my answer in my last post was wrong, reposted answer here is the right one.
Say
Empty pasture is full of grass in p days. Then rate of grass re-growth is 1/p pasture per day.
Cow eats full pasture in c days, so in 1 day it eats 1/c grass.
Goat eats full pasture in g days, so in 1 day it eats 1/g grass.
Duck eats full pasture in d days, so in 1 day it eats 1/d grass.
Therefore
Cow + goat eat (1/c + 1/g) pasture in 1day.
But in 1 day 1/p grass re-grows.
So cow + goat finish (1/c + 1/g) – 1/p grass in 1 day.
It is given that cow + goat eat Full + re-grown grass in 45 days. So in 1 day they finish 1/45 pasture.
1/c + 1/g – 1/p = 1/45 ……………………………………………………………………………………………………….. [1]
Similarly 1/c + 1/d - 1/p = 1/60 ….………………………………………………………………………………..……. [2]
Similarly 1/g + 1/d - 1/p = 1/90 ….……………….………………………………………………….………..……….. [3]
Also it is given that the amount of grass cow eats equals the amount of grass goat and duck eat together.
So 1/c = 1/g + 1/d ………………….………………………………………………………………………………………… [4]
From equations [3] & [4] 1/c – 1/p = 1/90 .………………………………………………………………….….… [5]
Hence from [1] & [5],
1/g + 1/90 = 1/45 or 1/g = 1/45 + 1/90 or 1/g = 1/90 ……………………………………………………….. [6]
Similarly from [2] & [5], 1/d = 1/ 180 ..……………………………………………………………………………… [7]
From equation [4], [6], & [7], 1/c = 1/60 …………..………………..…………………………..……………….. [8]
And from [5] & [8], 1/p = 1/180 ……………………………………………………………………………………….. [9]
Hence when all three eat the grass together, they finish 1/60 + 1/90 +1/180 – 1/180 = 1/36 grass in 1 day.
So all three cow + goat + duck together finish the grass in 36 days.
I used 'spoiler'. But somehow it did not work....?