DRDO Multi tasking staff Model paper 4

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.