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  • 1 Post By deesuwalka
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February 24th, 2017, 01:06 AM   #1
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another speed problem

Al and Bob, who live in North Vancouver, are Seattle Mariners fans. They regularly drive the 264 km from their home to the ballpark in Seattle. On one particular day, Bob drove to the game. On the return journey, Al was able to increase their average speed by 10% and save 18 minutes on the travelling time.

Calculate the average speed when Bob drove to the game.

This is how I have attempted it.

Let x be the time Bob took to drive to the game and let x-18 be the time Al took to drive back home.

264/x=(0.1)(264)/(x-eighteen)

Solving the equation gives x=20. But the answer key says x=80.

I don't get it. Please explain how to do it. Thanks.

Last edited by davedave; February 24th, 2017 at 01:10 AM.
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February 24th, 2017, 02:27 AM   #2
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Your attempt is wrong. Have a look at the following steps.

Let the speed at which Bob drive = $x \;km/p$

Speed of Al will be = $x+10\%$ of $x$

$=x+0.1x\implies1.1x$

Time saved by Al = $18\;min=\dfrac{18}{60}=0.3h$

Time is taken by Bob - Time is taken by Al = $0.3$

$=\dfrac{264}{x}-\dfrac{264}{1.1x}=0.3$

$1.1(264)-264=0.33x$

$26.4=0.33x$

$x=80$
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February 24th, 2017, 07:02 AM   #3
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b = Bob's speed

264/b - 264/(1.10b) = 18/60

Carry on...
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February 24th, 2017, 11:14 PM   #4
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Quote:
Originally Posted by Denis View Post
b = Bob's speed

264/b - 264/(1.10b) = 18/60

Carry on...
Easy on the speed Denis
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February 25th, 2017, 05:27 AM   #5
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Quote:
Originally Posted by Joppy View Post
Easy on the speed Denis

Worry none J, I'll save you some...
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February 26th, 2017, 12:02 AM   #6
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Thanks a lot, deesuwalka.
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