Round Trip

Stage: 3 Short Challenge Level: Challenge Level:1
See all short problems arranged by curriculum topic in the short problems collection

I walk to the bike shop at 3 miles per hour, and I cycle home at 12 miles per hour. What is my average speed, in miles per hour, for the time that I am actually on the route?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.  

This problem is taken from the UKMT Mathematical Challenges.