Answer: 6 km/hr
Let the speed of the current be \(x\) km/hr. Downstream speed \(= 12 + x\), upstream speed \(= 12 - x\).
\[\frac{36}{12 + x} + \frac{36}{12 - x} = 8\]\[36(12 - x + 12 + x) = 8(144 - x^2)\]\[864 = 1152 - 8x^2\]\[x^2 = 36\]\(x = 6\) (speed is positive). Check: \(\frac{36}{18} + \frac{36}{6} = 2 + 6 = 8\) hours.