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Kelly drove north for 9 miles and then east for 12 miles at an average rate of 42 miles per hour to arrive at the town of Prime. Brenda left from the same location, at the same time, and drove along a straight road to Prime at an average rate of 45 miles per hour. How many minutes earlier than Kelly did Brenda arrive?

 Jun 17, 2018
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Kelly drove  9 +12 miles =21 miles

This took this amount of time:

  21 miles/  42miles/hr = 1/2 hr = 30 minutes

 

Brenda drove  sqrt(9^2 + 12^2) = 15 miles

 This took this amount of time:

   15 miles / 45 miles/hr = 1/3 hr = 20 minutes

 

So Brenda arrived  30 -20 = 10 minutes before Kelly

 Jun 17, 2018

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