Bike Shop

Answer: $4.8$ miles per hour


Using ratio
At 3 mph, the journey takes 4 times longer than at 12 mph.
Return journey: one unit of time, 12 mph
Outward journey: 4 units of time, 3 mph, 3 mph, 3 mph, 3mph
Average speed = (12 + 3 + 3 + 3 + 3)$\div$5 = 4.8


Using algebra
Suppose the distance to and from the bike shop is $x$ miles.
Then the time taken on the journey there is $\frac{x}{3}$ hours, and the time taken on the journey back is $\frac{x}{12}$ hours.

So altogether a distance of $2x$ miles is travelled in $\frac{x}{3} + \frac{x}{12} = \frac{5x}{12}$ hours.

So the average speed is $2x \div \frac{5x}{12} = 4.8$ miles per hour.