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John is participating in a 180 kilometer bike race. John [#permalink]
16 Oct 2005, 07:30
100% (01:03) correct
0% (00:00) wrong based on 1 sessions
John is participating in a 180 kilometer bike race. John
rides at an average speed of 30 kilometers per hour for the
first 60 kilometers, 20 kilometers per hour for the next 60
kilometers, and 10 kilometers per hour for the final 60
kilometers. What was John's average speed for the race?
A. 15 km/h
B. 16 2/3 km/h
C. 22 1/2 km/h
D. 20 km/h
E. 16 4/11 km/h
The formula for rate is rate=distance/time. We already know that the distance of the race is 180 kilometers. We can determine the time it took John to finish the race from the information given in the problem. Using the formula for rate, we know that time = distance/rate. The time it took John to complete the first 60 kilometers is 60 km/(30 km/h) or 2 hours. The next 60 kilometers took him 60 km/(20 km/h) or 3 hours. The last 60 kilometers took him 60 km/(10 km/h) or 6 hours. Thus, it took John 11 hours to finish the race. Now we know the value for time in the rate formula. Solving for rate: