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Three machines, A, B, and C, can complete a certain task in 10 hours,

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Three machines, A, B, and C, can complete a certain task in 10 hours, [#permalink]

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Three machines, A, B, and C, can complete a certain task in 10 hours, 4 hours, and 5 hours respectively. If all 3 machines worked together for 1 hour and then stop, how many hours does it take machine C to complete the job?

A. 4/5
B. 9/4
C. 11/4
D. 4
E. 13/2
[Reveal] Spoiler: OA

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Re: Three machines, A, B, and C, can complete a certain task in 10 hours, [#permalink]

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In one hour three machines can complete (1/10 + 1/4 + 1/5) = 11/20 of the work

Work remaining = 9/20

(1/5)*x = 9/20
x = 9/4

Answer: B
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 [#permalink]

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New post 04 Apr 2017, 07:21
The first machine(A) completed 10% ( 1/10 meaning 10 hours is 100% completion so 1 hour is 10% of it.

The second machine (B) completed 25% (1/4 meaning 4 hours is 100% completion so 1 hour is 25% completion )

The third machine (C) completed 20% (1/5)

Therefore, total completion is 10% + 25% + 20% = 55%
Percentage left for completion is 45% to be done by machine C
We can now use proportion.

100% ---- 5 hours
45% ---- X hours

Where X means unknown. )
(Consequently, we cross multiple the above)
100X = 45x5

100X = 225

X = 225/100 X = 9/4 or 2.25 or. 2 1/4

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  [#permalink] 04 Apr 2017, 07:21
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