Question

A man can row at 5 km/hr in still water. If the river is running at 1 km/hr, it takes him 75 minutes to row to a place and back. How far is the place? 

(a) 2.5 km 

(b) 3 km 

(c) 4 km 

(d) 5 km

Answer 1

(b) 3 km

Let the distance to the place be x km. 

Speed downstream = (5 + 1) km/hr = 6 km/hr 

Speed upstream = (5 – 1) km/hr = 4 km/hr

Given, \(\frac{x}{6}+\frac{x}4=\frac{75}{60}\) hrs ⇒ \(\frac{2x+3x}{12}=\frac54\)

\(\frac{5x}{12}=\frac54\) ⇒ x = 3 km.