If 10 sheep are turned out in the grass field, the grass will be gone in 20 days;
On the other hand, if 15 sheep are turned out in the field, the grass will be gone in 10 days.
If 25 sheep are turned out in the field, when will the grass be gone?
(assuming each sheep eat grass at the same rate; and the grass is growing at constant rate.)

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参考答案：Let G be the total amount of grass in the field before the sheep are turned out;
Let R be the daily growing amount of grass;
Let M be amount of grass each sheep eats per day.

Two equations can be constructed:


Total amount of grass eaten = G + 20R = 20(10M);

G + 10R = 10(15M)

Reducing the equations:

G + 20R = 200M and G + 10R = 150M

Subtract one equation from the other:

10R = 50M So R = 5M.

Substituting back, G = 100M.

Now construct an equation for the case of having 25 sheep turned out in the field, where X is the number of days it takes the sheep to eat all the grass:


G + XR = X(25M) 100M + X(5M) = X(25M)

20XM = 100M

>>> X = 5

So 25 sheep would consume all the grass in the field in 5 days.

[ 本帖最后由 alloncrazy 于 2008-4-23 23:32 编辑 ]

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英语题目是不是春节期间三倍奖励啊？

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又是英文又是数学的