The motor boat moved along the lake at a speed of V0 = 18 km / h. Assuming that the water resistance force is proportional to the square of the speed Fsopr = rV ^ 2, determine the proportionality coefficient r, if after 30 seconds after turning off the motor the speed of the boat decreased to V = 9 km / h. The mass of the boat is m = 900 kg.

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