Correct option is B
Given:
Speed of stream = 6 km/h
Downstream distance = 56 km
Upstream distance = 39 km
Total time = 7 hours
Formula Used:
Downstream speed = boat speed + stream speed
Upstream speed = boat speed − stream speed
Total time = time downstream + time upstream
Time = SpeedDistance
Solution:
Let speed of boat in still water = x km/h
Then,
Downstream speed = x + 6
Upstream speed = x – 6
Now,
x+656+x−639=756(x−6)+39(x+6)=7(x+6)(x−6)56x−336+39x+234=7(x2−36)95x−102=7x2−2527x2−95x−150=0x=2×795±(−95)2−4×7×(−150)x=1495±9025+4200=1495±13225 x=1495+115=14210=15 x=1495−115=14−20=−710(discard negative)
The speed of the boat in still water is 15 km/h.