Question 206088
sol:
speed=distance/time
according to the question
speed of upstream=distance/time=12/3=4km/hr
according to the question speed of boat in still water=9 km/hr
let speed of stream= x km/hr
now speed of upstream=speed of boat in still water-speed of stream
substitute above values in this formula ,we get
4km/hr=9 km/hr-x
transposing x from R.H.S to L.H.S and 4 from L.H.S to R.H.S
x=9 km/hr-4 km/hr= 5km/hr
speed of stream=5 km/hr
check:speed of upstream=4km/hr
speed of upstream=speed of boat in still water-speed of stream
                  =9 km/hr- 5 km/hr=4 km/hr
L.H.S= R.H.S (as requried)