Question 321989
a boat can go 9 km. downstream in 45 mins.
 return 9 km in an hour. 
let speed of boat in still water be x mph
speed of current be y mph
..
9/ x+y= 3/4
36=3(x+y)
12 = x+y......... equation 1
9/x-y= 1
x-y=9.............equation 2
Add the two equations 
x+y+x-y=12+9
2x=21
x=10.5 mph
..
x-y=9
plug the value of x
10.5-y=9 
y=1.5 mph