Question

Ling lives 2 miles from school. It took him 15 minutes to bike from school to home. The first half of the distance he biked at a speed of 12 mph. What was his speed for the remaining distance? What was his average speed?

Answer 1

He was going 6 mph for the remaining distance 
the average is 9

Answer 2

1 mile at 12 m/h is .2 m/min
1 mile at .2 m/min (1/.2) = 5 min So the first mile took 5 minutes
To find how fast the second mile was:
You know it was 1 mile and it took 10 minutes
1/10=.1 m/min which is 6 m/h
The average speed is 6+12/2=9 m/h

Hope that helps.  Feel free to ask any questions.