A boat can travel 21 km in 3 hours going upstream . It can travel 55 km in 5 hours going downstream . What is the speed of the stream ?

Given,
Distance travelled in upstream = 21km
Time taken in upstream = 3hours
Let, Speed of the upstream = y kmph ( Speed * Time = Distance)
y kmph = 21/3 = 7 kmph

Given,
Distance travelled in downstream = 55km
Time taken in downstream = 5 hours
Let, Speed of the downstream = x kmph
x kmph = 55/5 = 11 kmph

Speed of the stream = 1/2(speed of the downstream - speed of the upstream)
= 1/2(x-y)
=1/2(11-7) = 2
Speed of the stream = 2 kmph