A ship sails out to a mark at the speed of 15 km/h and sails back to the starting point at the speed of 10 km/h. The average speed of sailing of the ship is:

12 km/h 15 km/h 13 km/h 14 km/h

12 km/h

Average Speed = \(\frac{2× S1 × S2 }{S1  +  S2}\) = \(\frac{2× 15 × 10 }{15  +  10}\) = \(\frac{300 }{25}\) = 12 km/h