A man can row upstream at 7 km/h and downstream at 11 km/h. What is the man's rate in water?

9 km/h

Man's rate in water means speed of man. The speed will be same as that of speed of boat as he is sitting on the boat

Let,

Speed of boat in still water= x

Speed of stream/current = y

According to the question:

Upstream speed (Us) = 7 km/h i.e. x - y = 7 km/h -----1st equation

Downstream speed (Ds) = 11 km/h i.e. x + y = 11 km/h -----2nd equation

Adding both the equations:

x - y + x + y = 7 + 11 

2x = 18

x = 9 km/h