math
posted by Lee .
18. A cyclist bikes x distance at 10 miles per hour and returns over the same path at 8 miles per hour. What is the cyclist's average rate for the round trip in miles per hour?

time for first trip = x/10
time for second trip = x/8
total time = x/10 + x/8
avg rate = total distance/total time
= 2x/(x/10 + x/8)
simplify, your x's should cancel out.
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