# The speed of movement of the ship relative to the coast down the river is 21 km / h, and up – 11 km / h

The speed of movement of the ship relative to the coast down the river is 21 km / h, and up – 11 km / h. Determine the speed of the current relative to the shore.

Let us derive the formula for finding the required current velocity relative to the coast: Vt = V1 – Vheat = Vheat – V2, from where we express: Vheat = (V1 + V2) / 2 and Vt = V1 – (V1 + V2) / 2.

Variables: V1 – speed of the ship down the river (V1 = 21 km / h); V2 – speed up the river (V2 = 11 km / h).

Let’s calculate: VT = V1 – (V1 + V2) / 2 = 21 – (21 + 11) / 2 = 21 – 16 = 5 km / h.

Check (we use a ready-made formula): Vt = (V1 – V2) / 2 = (21 – 11) / 2 = 5 km / h (correct).

Answer: The current speed is 5 km / h.

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