A boat can cover 120 km upstream and back in a total of 30 hours, and 25 km upstream and 40 km downstream in a total of 7 hours. How much distance will the boat cover in 16 hours in still water?

The boat covers 120 km upstream and back downstream in 30 hours.

The boat covers 25 km upstream and 40 km downstream in 7 hours.

Speed of boat in still water: b (km/h)

Speed of the current: c (km/h)

Speed of the boat upstream, U = b − c (km/h)

Speed of the boat downstream, D = b + c (km/h) 

multiplying by 3 in equation (ii) and then subtracting both (i) and (ii) .

​$$\frac{120}{\text{U}} + \frac{120}{\text{D}} = 30 \space .... \text {(i)} \\ \ \\ \underline{ -\frac{75}{\text{U}} - \frac{120}{\text{D}} = -21} \space .... \text{(ii)} \\ \ \\ \text { we get} \space \frac{ 45}{\text{U}} = 9$$

Now Upstream, U ( b - c ) = 5 km/h 

So, Downstream, D ( b + c )  = 20 km/h 

So the distance cover in 16 hours by boat  = $$\frac{ 20 + 5}{2} \times 16 = 200 km$$ ​