CAT 2017 Quantitative Ability

Let the speed of the stream be 'r' kmph.The onward journey, A to B, is an upstream one and the effective speed = 10 - r.
The return journey, B to A, is a downstream one and the effective speed = 10 + r
The trip from A to B takes 2 hours and 40 minutes more than the return trip from 8 to A 2 hours and 40 minutes is equal to $\frac{8}{3}$ hours.
Or we have $\frac{20}{10-\mathit{r}}=\frac{20}{10+\mathit{r}}+\frac{8}{3}$
Solving for r, we get r = 5 kmph.
For the second case let us assume the still water speed of the boat to be 'b' kmph
The onward journey, A to B, is an upstream one and the effective speed = b - 5
The return journey, B to A, is a downstream one and the effective speed = b + 5.
We also know that the onward trip takes 2 hours more than the return trip.

$\frac{20}{\mathit{b}-5}=\frac{20}{\mathit{b}+5}+2$
Solving for b, we get
b =