(13) The speed of a sub marine over sea surface is 30 knot per hour and below sea surface is 20 knots per hour. What is the average speed of the submarine if it is below water surface for 3/4 th of her time of journey ? What is the average speed of the submarine if it is under water surface for 3/4 th of the total distance covered by him?
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sorry i am very sorry i dont know
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