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Average Speed		  
Q Louise runs the first half of a race at 5 miles per hour. Then she picks up her pace and runs the last half of the race at 10 miles per hour. What is her average speed on the course?


  
A The answer isnot7.5 miles per hour. Assume the course is 20 miles long. Louise needs 2 hours to run the first 10 miles (the first half of the course) and 1 hour to run the last 10 miles (the last half), meaning she needs 3 hours to run the total of 20 miles. Therefore, her average speed is 6.7 (or 6 2/3) miles per hour.

Source: Carter & Russell,Classic Brain Puzzlers,p.57