Correct Answer - Option 3 : 1.25 km/hr
Given:
Distance (D) = 20 km
TD (Time taken in downstream) = 4 hours
TU (Time taken in upstream) = 8 hours
Formula used:
Downstream speed (SD) = Speed of boat + Speed of stream
Upstream speed (SU) = Speed of boat – Speed of stream
TD = DD/SD
Where TD, DD, and SD are time, distance and speed respectively during downstream
TU = DU/SU
Where TU, DU, and SU are time, distance and speed respectively during upstream
Calculation:
Let the speed of boat be x km/hr and speed of stream = y km/hr
⇒ SD = x + y
⇒ SU = x – y
TD = D/SD
⇒ TD = D/(x + y)
⇒ 4 = 20/(x + y)
⇒ x + y = 5 ---- (1)
TU = D/SU
⇒ TU = 20/(x – y)
⇒ 8 = 20/(x – y)
⇒ x – y = 2.5 ---- (2)
Solving (1) and (2)
⇒ x = 3.75 km/hr and y = 1.25 km/hr
∴ Speed of stream is 1.25 km/hr