Given, Distance =150km Speed of motor boat (x)=20km∕h Total time =16h Let Speed of flow of river (y)= ? When boat move downstream =20+y When boat move upstream =20−y Speed =
Distance
Time
Time =
Distance
Speed
⇒16=
150
(20+y)
+
150
(20−y)
⇒16=
150(20−y)+150(20+y)
(20)2−y2
⇒16×202−16y2 =150×20−150y+150×20+150y ⇒16×400−16y2=3000+3000 [∵(a−b)(a+b)=a2−b2] ⇒16y2=6400−6000 ⇒16y2=400 ⇒y2=25 ⇒y=5km∕h Speed of flow of the river =5km∕h.