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PS:average speed

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Joined: 16 Jan 2009
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Schools: Anderson '15, Insead '13
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PS:average speed [#permalink] New post 01 Jul 2009, 00:07
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Question Stats:

0% (00:00) correct 100% (00:00) wrong based on 0 sessions
A bus trip of 450 miles would have taken 1 hour less if the average speed S for the trip had been greater by 5 miles per hour. What was the average speed S, in miles per hour, for the trip?
(A) 10
(B) 40
(C) 45
(D) 50
(E) 55
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Lahoosaher

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Re: PS:average speed [#permalink] New post 01 Jul 2009, 00:36
Time=distance/avg.speed.
Time1=450/S,
Time2=Time1-1=450/(S+5)
Gathering both together
450/(S+5)=450/S-1
450/(S+5)=(450-S)/S
450*S=(450-S)*(S+5)
450S=450S+2250-S^2-5S
S^2+5S-2250=0
Solving the root
(-5+95)/2= 45
S=45
Time1=10
s+5=50
Time2=9
;)
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Re: PS:average speed [#permalink] New post 02 Jul 2009, 11:45
R*T = D

Slower trip (1 hour more):
S * (T + 1) = 450

Faster trip (1 hour less):
(S + 5) * T = 450

S*(T+1) = (S+5)*T
ST + S = ST + 5T
S = 5T

Plug in this value of S into the first equation:
5T*(T+1) = 450
T*(T+1) = 90
T = 9

S = 5T = 5*9 = 45

C
Re: PS:average speed   [#permalink] 02 Jul 2009, 11:45
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