SOLUTION: A car travels 40 mph for 20 miles, 36 mph for 24 miles, and 48 mph for 16 miles. what is its average speed?

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Question 178125: A car travels 40 mph for 20 miles, 36 mph for 24 miles, and 48 mph for 16 miles. what is its average speed?
Answer by josmiceli(9695) About Me  (Show Source):
You can put this solution on YOUR website!
The average speed is NOT the average of the averages
The average speed is (total distance)/(total time)
For the 3 parts od the trip,
(1) d%5B1%5D+=+r%5B1%5D%2At%5B1%5D
(2) d%5B2%5D+=+r%5B2%5D%2At%5B2%5D
(3) d%5B3%5D+=+r%5B3%5D%2At%5B3%5D
given:
(1) 20+=+40%2At%5B1%5D
(2) 24+=+36%2At%5B2%5D
(3) 16+=+48%2At%5B3%5D
----------------------
t%5B1%5D+=+1%2F2
t%5B2%5D+=+2%2F3
t%5B3%5D+=+1%2F3
t%5B1%5D+%2B+t%5B2%5D+%2B+t%5B3%5D+=+1%2F2+%2B+2%2F3+%2B+1%2F3
t%5Bsum%5D+=+3%2F6+%2B+4%2F6+%2B+2%2F6
t%5Bsum%5D+=+9%2F6
t%5Bsum%5D+=+3%2F2 hrs
and
d%5B1%5D+%2B+d%5B2%5D+%2B+d%5B3%5D+=+20+%2B+24+%2B+16
d%5Bsum%5D+=+60
Average speed = d%5Bsum%5D%2Ft%5Bsum%5D+=+60%2F%283%2F2%29
60%2F%283%2F2%29+=+%282%2F3%29%2A60
%282%2F3%29%2A60+=+40 mi/hr answer