One Sheep, One Goat and One Cow
Level
pending
A sheep and a goat eat a stack in 90 days. The sheep and a cow eat this stack in 60 days. The cow and the goat eat this stack in 45 days. How many days the sheep, the goat and the cow need to eat this stack?
5
20
10
40
42
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1 solution
The sheep and the goat eat 1/90 of the stack in a day, the sheep and the cow eat 1/60 of the stack in a day and the cow and the goat eat 1/45 of the stack in a day. So 1/90 + 1/60 + 1/45 = 9/180 = 1/20. We have the the sheep and the goat, the sheep and the cow, the cow and the goat - 6 animals. If this 6 animals, 2 cows, 2 goats, 2 sheep eat 1/20 of the stack in a day, they will eat this stack in 20 days. But we need 3 animals, 1 cow, 1 sheep and 1 goat, so this 3 animals will eat this stack in 2*20=40 days. Hence, this is the answer, 40 days.