Question 1121502
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<pre>
The effective speed downstream is the sum of speeds


    21 km/h + 9 km/h = 30 km/h.        <<<---=== it is the speed relative to the river banks.


The time to travel 60 km downstream is  {{{60/30}}} = 2 hours.
</pre>

<U>Answer</U>.  2 hours.