Let the speed of boat in still water=x km/hr

speed of stream (current)=2 km/hr

Then,

speed of boat in up stream=(x-2)km/hr=s1

speed of boat in downstream=(x+2)km/hr=s2

distance travel upstream=d1=10km

distance travel downstream=d2=10km

t1=d1/s1=10/((x-2)

t2=d2/s2=10/(x+2)

Total time taken=t=55min=55/60hr=11/12hr

t1+t2=t

[10/(x-2)]+[10/(x+2)]=11/12

x=22 0r x=-(2/11)

negative speed is not possible

so x=22km/hr

Ans :Speed of boat in still water is 22km/hr