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

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The time it took car A to travel 400 miles was 2 hours less [#permalink] New post 23 Nov 2006, 00:57
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 A’s average speed was 10 miles per hour greater than that of car B, what was car B’s average speed, in miles per hour? (A) 20 (B) 30 (C) 40 (D) 50 (E) 80

can easily be found out by backsolving....but whats the best and fastest way of doing this?
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 [#permalink] New post 23 Nov 2006, 01:06
You can solve this one quickly by picking numbers....

Choose answer choices and plug them into formula:

A: r*t=400; choose 40; 40t=400.....t=10

B: (r+10)(t-2)=d; plug in r=40, t=10..... (40+10)(10-2)=50*8=400... that is what we wanted to see....
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 [#permalink] New post 23 Nov 2006, 02:13
this is what i do though:

u get 2 equations:
400/a + 2 = 400/b
a = b + 10.

then u get this equation:
b^2 + 10b = 2000

then plug in C first. Bingo. Next and confirm. :-D


Directly plugging in may be very confusing.
  [#permalink] 23 Nov 2006, 02:13
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