Given that, time taken to travel upstream = 2 × time taken to travel downstream

When distance is constant, speed is inversely proportional to the time

Hence, 2 × speed upstream = speed downstream

Let speed upstream $=x$

Then speed downstream $=2x$

we have, $\dfrac{1}{2}(x + 2x)$ = speed in still water

$\Rightarrow \dfrac{1}{2}(3x) = 7.5$

$\Rightarrow 3x = 15$

$\Rightarrow x = 5$

i.e., speed upstream = 5 km/hr

Rate of stream $=\dfrac{1}{2}(2x-x) = \dfrac{x}{2}= \dfrac{5}{2} = 2.5\text{ km/hr}$