A boat can cover 36 km downstream and 16 km upstream in 10 hours. It can cover 3 km downstream and 1.2 km upstream in 48 minutes. What is the speed (in km/hr) of the boat when going upstream?

let the speed of the boat=b
speed of the stream=s
Given (36)/(b+s) +(16)/(b-s) =10
(36b-36s+16b+16s)/((b-s)(b+s))=10
52b-20s=10(b-s)(b+s)
Also given (3)/(b+s) +(1.2)/(b-s) =4/5
(3b-3s+1.2b+1.2s)/((b-s)(b+s))=4/5
4.2b-1.8s=(4/5)(b+s)(b-s)
dividing both the equations we have
 104b-40s=105b-45s
-b=-5s
b=5s
Substituting in the above equation
(3)/(5s+s) +(1.2)/(5s-s) =4/5
(1/2s)+(3/10s)=4/5
8/10s =4/5
s=1 km/hr
b=5 km/hr
Upstream speed=5-1
=4 km/hr