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# the time it took car A to travel 400 miles was 2 hours less

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VP
Joined: 30 Sep 2004
Posts: 1488
Location: Germany
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the time it took car A to travel 400 miles was 2 hours less [#permalink]  09 Jan 2005, 07:21
the time it took car A to travel 400 miles was 2 hours less than the time it took car B to travel the same distance. if car As average speed was 10 miles per hour greater than that of car B, what was car Bs average speed, in miles per hours ?

a 20
b 30
c 40
d 50
e 80

backsolve is easy and preferable, but can someone please tell me the equation for it. thanx !!!
Intern
Joined: 15 Dec 2004
Posts: 25
Followers: 1

Kudos [?]: 2 [0], given: 0

ok..ans is (C), and backsolving is the best way.

but any how

lets time taken by b is t hrs and its speed is x.

x * t = 400
(x+10)*(t-2) = 400

solving for x. replacing t = 400/x into second equation:

x2 (x square) + 10x -2000 = 0

(x+50) (x -40)=0

x = -50 & x=40. since speed cannot be negative, chuck -50.
Director
Joined: 07 Jun 2004
Posts: 614
Location: PA
Followers: 3

Kudos [?]: 316 [0], given: 22

i did it this way

since B takes 2 hours more then Time by B - Time by A = 2

so 2 = 400/v - 400/ v+10

if you solve it we get V = 40 or -50 so V =40
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