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-   -   How to solve this confusing question? (http://mymathforum.com/elementary-math/344510-how-solve-confusing-question.html)

Ganesh Ujwal July 4th, 2018 01:06 AM

How to solve this confusing question?
 
100 cows can graze a field completely in 15 days, while 60 cows can do it in 30 days. How many cows are needed to do it in 10 days?

Answer is 140 cows

I tried: It'd seem like it'd only take 50 cows to graze a field in 30 days if 100 can do it in 15. But in question it shows 60 cows can do it in 30 days. I am confused with this question.

weirddave July 4th, 2018 02:53 AM

You have to take account of the grass growing during the grazing time.

skipjack July 4th, 2018 02:55 AM

Let c be the fraction of a field that a cow grazes in a day.

Let g be the fraction of a field that grows in a day, and assume that g is a constant.

You can now write two equations: 15(100c - g) = 1 and 30(60c - g) = 1.

Solving those gives c = 1/1200 and g = 1/60.

If n cows can graze the field completely in 10 days, 10(n/1200 - 1/60) = 1,
so n = 120(1 + 1/6) = 140.

jonah July 4th, 2018 02:58 AM

Beer soaked counter questions follow.
Quote:

Originally Posted by Ganesh Ujwal (Post 596194)
100 cows can graze a field completely in 15 days, while 60 cows can do it in 30 days. How many cows are needed to do it in 10 days?

Answer is 140 cows

I tried: It'd seem like it'd only take 50 cows to graze a field in 30 days if 100 can do it in 15. But in question it shows 60 cows can do it in 30 days. I am confused with this question.

Try something solving something funnier like the the following two questions; believe it or not, it will straighten you up.

A man can drink a cask of wine in 20 days, but if his wife drinks with him it will take only 14 days—how long would it take for the wife alone?

If one priest can pray a soul out of purgatory in 5 hours, while it takes a second priest 8 hours, how long will it take if the two priests prayed together?

Ganesh Ujwal July 4th, 2018 03:38 AM

Quote:

Originally Posted by skipjack (Post 596196)
Let c be the fraction of a field that a cow grazes in a day.

You can now write two equations: 15(100c - g) = 1 and 30(60c - g) = 1.

How did you write this two equations?

why 1 after "equal to" sign ? why not 2,3, 50,100?

skipjack July 4th, 2018 04:19 AM

The "1" is because 1 field is grazed in the 15 days (or 30 days for the second equation).

Ganesh Ujwal July 4th, 2018 04:40 AM

Quote:

Originally Posted by skipjack (Post 596199)
The "1" is because 1 field is grazed in the 15 days (or 30 days for the second equation).

Why we need to consider grass growing during the grazing time?

Benit13 July 4th, 2018 05:48 AM

Quote:

Originally Posted by Ganesh Ujwal (Post 596200)
Why we need to consider grass growing during the grazing time?

Well, like you said in your first post, you were expecting that 50 cows would be needed to graze the field in 30 days, but that's not the case. Instead, it is 60. What reasons could there be for needing more cows to graze the field?

Denis July 4th, 2018 06:09 AM

As an aside: if grass grows constantly, field will never be "completely" grazed!

skipjack July 4th, 2018 09:17 AM

Every cow was Kung Fu grazing,
Those cattle were faster than lightning!
In fact, it was a little bit frightening...
But they mowed with expert timing!


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