SOLUTION: you bicycle 15 miles at 21 mph, then 20 miles at 18 mph, and finally 30 miles at 12. how do i find the average speed? do i just add each of the speeds and divide by 3? or is there

Algebra ->  Customizable Word Problem Solvers  -> Travel -> SOLUTION: you bicycle 15 miles at 21 mph, then 20 miles at 18 mph, and finally 30 miles at 12. how do i find the average speed? do i just add each of the speeds and divide by 3? or is there       Log On

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Question 813170: you bicycle 15 miles at 21 mph, then 20 miles at 18 mph, and finally 30 miles at 12. how do i find the average speed? do i just add each of the speeds and divide by 3? or is there something else i need to do with the miles?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Average speed is always ( total distance traveled ) / ( total time spent traveling )
It is not an average of several average speeds
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1st part of trip:
+15+=+21t+
+t+=+15%2F21+
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2nd part:
+20+=+18t+
+t+=+20%2F18+
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3rd part:
+30+=+12t+
+t+=+30%2F12+
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The total distance traveled is +15+%2B+20+%2B+30+=+65+ mi
The total time is +5%2F7+%2B+10%2F9++%2B+5%2F2+ hrs
You can find the common denominator, or just
use decimal equivilants
+.71429+%2B+1.111+%2B+2.5+
+t+=+4.3254+ hrs
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+d%2Ft+=+65%2F4.3254+ mi/hr
+d%2Ft+=+15.0275+
The average speed is 15.0275 mi/hr
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