Sarah takes 3 hours to climb the hill, and $1\frac{1}{2}$ hours to get down.
So altogether her journey takes $4\frac{1}{2}$ hours.

In this time she travels $12$ miles, so her average speed is
$12 \div 4\frac{1}{2} = 12 \times \frac{2}{9} = 2\frac{2}{3}$ miles per hour.

See all short problems arranged by curriculum topic in the short problems collection
This problem is taken from the UKMT Mathematical Challenges.