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by [email protected] » Tue Dec 23, 2008 6:16 pm
Q) Six machines, each working at the same constant rate, together can complete a certain job in 12 days. how many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?

A)2
B)3
C)4
D)6
E)8
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by amitabhprasad » Tue Dec 23, 2008 6:21 pm
I am getting "B"
12 days 6 machine
1 machine = 72 days
8 days = 72/8 = 9 machine
delta = 3 machine
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That's correct

by Ossa » Tue Dec 23, 2008 6:37 pm
Remember that in questions like this you can add the rates, so in the first case, quantity/6r=12 and in the second case, quantity/xr=8. Accordingly x=9, and so the additional number of machines=3.

amitabhprasad wrote:I am getting "B"
12 days 6 machine
1 machine = 72 days
8 days = 72/8 = 9 machine
delta = 3 machine
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by parallel_chase » Tue Dec 23, 2008 8:42 pm
Here is a simple way to remember things

work = rate * time * no. of machines

let work be 1

1 = rate * 12 * 6

rate = 1/72

everything remains the same except for no. of machines

work = rate * time * no. of machines

1 = 1/72 * 8 * no. of machines


no. of machines = 72/8 = 9

additional machines required = 9-6 = 3

Hope this helps.
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