Correct Answer - Option 1 : 30 km
Given:
Speed of boat in still water = 8 km/hr
Speed of stream = 2 km/hr
Difference of time taken during upstream and downstream = 2 hours
Concept used:
Downstream speed (SD) = Speed of boat + Speed of stream
Upstream speed (SU) = Speed of boat – Speed of stream
TD = DD/SD
TU = DU/SU
Calculation:
Let the distance travelled during upstream be D km
Distance travelled during downstream = D km
SD = 8 + 2 = 10 km/hr
SU = 8 – 2 = 6 km/hr
TD = D/SD
⇒ TD = D/10
TU = D/SU
⇒ TU = D/6
According to question
TU – TD = 2
⇒ D/6 – D/10 = 2
⇒ 2D/30 = 2
⇒ D = 30 km
∴ Distance travelled during downstream is 30 km