You may recognize the numbers as belonging to the (5, 12, 13) Pythagorean triple. The runners traveled 12 km west after traveling 5 km south. Their total distance was ...
 5 km +12 km = 17 km
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By the Pythagorean theorem, ...
 (straight-line distance)² = (distance south)² +(distance west)²
In km, this is
 13² = 5² +(distance west)²
 169 -25 = (distance west)²
 √144 = 12 = distance west