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### Question

The product of the downstream and upstream speeds of a boat in a river is 20. What is the difference between the squares of the speed of the boat in still water and the speed of the stream?

**A**

20.

**B**

22.

**C**

24.

**D**

28.

**Soln.**

**Ans: a**

Let the speed of the boat in still water be u, and let v be the speed of the stream. The product of the upstream and downstream speeds of the boat is $(u + v) × (u - v)$, which is $u^2 - v^2$, and, therefore, the required answer = 20.