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Some PS questions- Need explanation

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Some PS questions- Need explanation [#permalink] New post 17 Aug 2010, 05:18
Need help in explanation of following problems. Need to know how quickly we can solve such problems to save time.

3. Three printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take R, working alone at its constant rate, to do the same job?
A. 8
B. 10
C. 12
D. 15
E. 20

OA E
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Re: Some PS questions- Need explanation [#permalink] New post 17 Aug 2010, 08:38
udaymathapati wrote:
Need help in explanation of following problems. Need to know how quickly we can solve such problems to save time.

3. Three printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 hours. S and T, working together at their respective constant rates, can do the same job in 5 hours. How many hours would it take R, working alone at its constant rate, to do the same job?
A. 8
B. 10
C. 12
D. 15
E. 20

OA E



Three printing presses, R, S, and T, working together at their respective constant rates, can do a certain printing job in 4 ==> 1/R +1/S + 1/T = 1/4 (1)

S and T, working together at their respective constant rates, can do the same job in 5 hours

1/S + 1/T = 1/5 (2)

(1)-(2) ==> 1/R=1/5 - 1/4 = 1/20 ==> R=20

Hope it helps
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Re: Some PS questions- Need explanation [#permalink] New post 17 Aug 2010, 13:21
lets say time taken by r = r hrs
lets say time taken by s = s hrs
lets say time taken by t = t hrs

given to us = 1/r + 1/s + 1/t = 1/4 -----(1)
1/s + 1/t = 1/5 -----(2)

plug in (2) in (1)

1/5 + 1/r = 1/4

1/r = 1/20
r = 20 hrs
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Re: Some PS questions- Need explanation [#permalink] New post 17 Aug 2010, 14:37
1/R +1/S + 1/t = 1/4

given that 1/S +1/T = 1/5

that makes it
1/R + 1/5 = 1/4
thus 1/R = 1/4 -1/5 = 5-4 /20 =1/20
hence R = 20 .

Hope this helps :)

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Re: Some PS questions- Need explanation   [#permalink] 17 Aug 2010, 14:37
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Some PS questions- Need explanation

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