Given: Speed of the motor boat = 15 km/hr Calculations: Let speed of the stream = v Speed downstream = (15 + v) km/hr Speed upstream = (15 – v) km/hr Time taken downstream = 30/(15 + v) Time taken downstream = 30/(15 + v) Time taken downstream = 30/(15 + v) Total time =
30
15+v
+
30
15−v
=4.5 =
30
15+v
+
30
15−v
= 9/2 ⇒
1
15+v
+
1
15−v
=
9
60
=
3
20
⇒
30
152−v2
=
1
20
⇒225–v2=200 ⇒v2=225−200=25 ⇒ v = 5 km/hr ∴ The speed of the stream (in km/hr) is 5 km/hr.