A boatman can row his boat in still water at a speed of 9 km/h. He can also row 44 km downstream and 35 km upstream in 9 hours. How much time (in hours) will he take to row 33 km downstream and 28 km upstream?

7

Direct approach,

Here we can see that,  The Downstream distance in both cases (44 km & 33 km) are divisible by 11

The Upstream distance in both cases (35 km & 28 km) are divisible by 7.

We can say that because the time required (Number of hours) is a complete number in both cases. (9 hrs & all options are complete numbers not in decimal)

Here, according to the question, 

x = 9 km/hr.

= 44/(9 + y) + 35/(9 - y) = 9 hrs

Put y = 2, so that 9 + y will be 11 & 9 - y = 7

= 44/11 + 35/7 = 4 + 5 = 9

y = 2 satisfies the given equation,

Now, 

= 33/(9 + y) + 28/(9 - y) = 33/(9 + 2) + 28/(9 - 2)  

= 33/11 + 28/7 = 3 + 4 = 7 hrs