N
A (North)
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O-----------------B (East)
OAB is a right triangle with the hypotenuse AB = 100 km
Le x be OB, the distance travelled East, the distance OA travelled North
is x+20 (given)
Now apply Pythagoras:
AB² = OA² + OB² ↔ 100² = (x+20)² + x²
After expanding you'll find:
2x²+40x-9600 = 0. Solve this quadratic equation and you will find :
x' = 60 and x" = -80 (extraneous). Hence OB = x = 60 km
