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PS ,, AVG speed

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PS ,, AVG speed

by dabulaila » Mon Aug 20, 2007 11:50 pm
12. 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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by erdnah » Tue Aug 21, 2007 12:03 am
450/s=t
450/(s+5)=t-1

450/(s+5)=(450/s)-1
450=(450/s)(s+5)-(s+5)
450=(450s+2250)/s-(s+5)
450+s-5=(450s+2250)/s
445+s=(450s+2250)/s
445s+s²=450s+2250
s²-5s-2250=0

s=50
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by dabulaila » Tue Aug 21, 2007 3:19 am
your answer is wrong
here's the solution...

450/t=s
450/t-1=s+5

450/t=(450/t-1) - 5
450= (450/t-1)t - 5t
450= (450t/t-1) - 5t(t-1)/t-1
450= (450t-5t^2+5t)/ t-1
450(t-1)= 450t -5t^2 +5t
450t-450 - 455t +5t^2
5t^2-5t-450=0
t^2-t-90=0
(t+9)(t-10)=0
then t=10

450/t=s
450/10= 45
so the answer is C and not D 50
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by erdnah » Tue Aug 21, 2007 3:22 am
you're right! :roll:
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