# Solving for X problem?

• February 20th 2009, 07:30 AM
norifurippu
Solving for X problem?
Hello, I'm having trouble working with this problem:

A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

The correct answer is 4, but I don't understand how to get it.
I thought I had to do: 9 x 3 / 12, but it's not correct.

How can I solve this?

Thanks
• February 20th 2009, 07:49 AM
earboth
Quote:

Originally Posted by norifurippu
Hello, I'm having trouble working with this problem:

A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

The correct answer is 4, but I don't understand how to get it.
I thought I had to do: 9 x 3 / 12, but it's not correct.

How can I solve this?

Thanks

12 persons work 3 days.
then
1 person works 3 * 12 days
then
9 persons work $\dfrac{3 \cdot 12}9$ days.
• February 20th 2009, 07:54 AM
12 persons do work in 3 days
So
Work done in 3days =1
Hence work done in 1day by 12 persons = 1/3

This is work done by 12 pewrsons
So the work done by 4 persons = 9/(3x12) = 3/12
thus to do complete work

Time taken =12/3= 4 days
• February 20th 2009, 07:54 AM
HallsofIvy
Quote:

Originally Posted by norifurippu
Hello, I'm having trouble working with this problem:

A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

The correct answer is 4, but I don't understand how to get it.
I thought I had to do: 9 x 3 / 12, but it's not correct.

How can I solve this?

Thanks

Assuming every one works at the same rate, the each is working at a rate of (1/12) job in 3 days or 1/36 job per day. 9 people will work at 9/36= 1/4 job per day and so 4/1= 4 day per job: 4 days to do the job.

By the way: your answer, 9 x 3/12= 27/12 is slightly larger than 2 days and so less than 3 days. You are saying that 9 people can do the job faster than 12 people! That should hae told you that you should have divided where you
multiplied.
• February 20th 2009, 09:58 AM
stapel
Quote:

Originally Posted by norifurippu
A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

How many man-days are consumed by twelve people working three days each? (Hint: Multiply.)

If you divide that number of man-days between nine people, how many days would that require? (Hint: Divide.)

Have fun! :D
• February 20th 2009, 10:27 AM
norifurippu
Thank you very much everyone! at first it appeared like a very easy problem, but it ended up requiring quite an amout of thinking, thanks!