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March 11th, 2008, 05:21 AM  #1 
Newbie Joined: Mar 2008 Posts: 1 Thanks: 0  Distance, time and rate
To cover the distance of 180 km, with average speed 15 km/h less than expected, took 1 hour longer than expected. Find the average speed.

March 11th, 2008, 06:28 AM  #2 
Senior Member Joined: May 2007 Posts: 402 Thanks: 0 
If v is the average speed, then the time to cover 180km takes: t=180/v Now, you say that if we were going with a speed that's 15 km/h smaller than the average, we would cover the same distance in 1 hour more. So, we have: 180/(v15) = 180/v+1 180 v/(v15) = 180+v 180 v = 180(v15)+v(v15) 180 v = 180 v15*180+v(v15) 0= v^215v+15*180 Using the quadratic formula, we get: v1=45 v2=60 Since the first answer makes no sense, the answer is v=60. Checking: 180/(v15) = 180/v+1 180/(6015) = 180/60+1 180/(6015) = 3+1 4=4 
March 11th, 2008, 12:09 PM  #3 
Global Moderator Joined: Dec 2006 Posts: 20,754 Thanks: 2137 
No. The average speed was expected to be 60 km/h, but was actually 45 km/h.


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