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.