Let us say boat travel xkm distance from base in t1 hours down stream when it turn around and comes upstream.

`t1 = x/25`

For the return travel let us say it takes t2 hours.

`t2 = x/15`

How ever at the critical time limit;

`t1+t2 = 3`

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Let us say boat travel xkm distance from base in t1 hours down stream when it turn around and comes upstream.

`t1 = x/25`

For the return travel let us say it takes t2 hours.

`t2 = x/15`

How ever at the critical time limit;

`t1+t2 = 3`

`x/25+x/15 = 3`

`(x(3+5))/75 = 3`

`x = (75xx3)/8`

`x = 28.125`

`t1 = x/25 = 28.125/25 = 1.125`

*So the boat should turn around in 1.125 hours to return to base without running out of fuel.*