A delivery rider leaves the depot, rides 9 km south, turns right and rides 12 km. What is the straight-line distance from the rider to the depot?

Explanation

1. 01

   Right from south is west, so the legs (9 km south, 12 km west) are perpendicular.

2. 02

   Distance $= \sqrt{9^2 + 12^2} = \sqrt{81 + 144} = \sqrt{225} = 15$ km.