What Time Does She Get to Work?
Date: 04/30/2002 at 17:25:59 From: David Williams Subject: Distance vs. time Lillian drives to work at an average speed of 45 miles per hour. If she travels 30 miles to work and leaves at 8:00 A.M., what time does she get to work? Please help me - I really don't know what I'm doing.
Date: 05/01/2002 at 11:09:29 From: Doctor Ian Subject: Re: Distance vs. time Hi David, The first step in solving a problem is to identify what you want to know. In this case, you want to know when Lillian gets to work. You could figure that out if you knew how long it takes her to drive to work. So what you're _really_ trying to find out is how long it takes to drive 30 miles at 45 miles per hour. Now, let's say that it takes H hours to drive 30 miles at 45 miles per hour: H => 30 miles at 45 miles per hour If you drive twice as fast, it will take half as long: H/2 => 30 miles at 90 miles per hour If you drive 3 times as far, it will take three times as long: 3H/2 => 90 miles at 90 miles per hour But 90 miles at 90 miles per hour should take exactly one hour, right? So 3H/2 = 1 hour Now, there is a shorter way to do it, which is to use the formula distance = rate * time 30 miles = 45 miles/hour * ? hours and solve for the '?'. But the problem with formulas is that they're easy to forget, which is why it's good to know how to do it the other way, too. It's like knowing two ways to get home, in case one of them is blocked for some reason. Does this help? - Doctor Ian, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.