SSC Selection Post 2023 PYQ — Question 73

The correct answer is option 1 i.e. 5 km/h.

Speed of boat = 20 km/hr

and let the speed of the stream be y km/hr

Upstream speed = 20 - y

Downstream speed = 20 + y

Now, according to the question

⇒ ${75\over(20-y)}+{75\over(20+y)}$ = 8

⇒ $75({1\over20-y}+{1\over20+y})$ = 8

⇒ $75({20+y+20-y\over(20-y)(20+y)})$ = 8

⇒ $75({40\over20^2-y^2})$ = 8

⇒ ${75\times5\over20^2-y^2}$ = 1

⇒ 375 = 400 - y²

⇒ y² = 25

⇒ y = 5