a) 2 mph
b) 2.5 mph
c) 3 mph
d) 4 mph
Answer: a
Explanation: Let the speed of the stream x mph.
Speed downstream = (10 + x) mph
Speed upstream = (10 – x) mph
$$\eqalign{
& \frac{{36}}{{ {10 – x} }} – \frac{{36}}{{ {10 + x} }} = \frac{{90}}{{60}} \cr
& 72x \times 60 = 90\left( {100 – {x^2}} \right) \cr
& {x^2} + 48x – 100 = 0 \cr
& \left( {x + 50} \right)\left( {x – 2} \right) = 0 \cr
& x = 2\,\text{mph} \cr} $$`
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