IBPS SO Prelims Exam Model Paper 8

Given:
Upstream distance traveled = 75 km
Downstream distance traveled = 75 km + 20% of 75 = 90 km
Speed of boat = 20 km/h
Formula used:
Time :

distance

speed

Calculation:
Let the speed of stream be s km/h
According to question:
[

75

(20–s)

] = [

90

(20+s)

] + (

7

5

)
⇒ [

75

(20–s)

] – [

90

(20+s)

] = (

7

5

)
⇒

[75(20+s)–90(20–s)]

[(20)2–(s)2]

=

7

5

⇒

[1500+75s–1800+90s]

(400–s2)

=

7

5

⇒

(−300+165s)

(400–s2)

=

7

5

⇒ -1500 + 825s = 2800 – 7s2
⇒ −7s2 – 825s + 4300 = 0
On solving,
s = 5 km/h (neglecting negative value of ‘s’)
∴ Speed of stream is 5 km/h