(c) 5 : 1
Let the speed downstream be u km/hr and speed upstream be v km/hr.
Given, 2u = 3v ⇒ u = \(\frac{3v}{2}\)
Required ratio = \(\frac12(u+v):\frac12(u-v)\)
= \(\bigg(\frac{3v}{2}+v\bigg):\bigg(\frac{3v}{2}-v\bigg)\)
= \(\frac{5v}{2}:\frac{v}2=5:1.\)