||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