Well if I did the math treating Duncan's age as a variable rather than a constant, Kurk's age would be 15.57 years old, and it really doesn't make sense for anyone to describe their age with such precision as to delve into numbers with decimals, and him saying he's old enough as a 17 year old makes a lot more sense than him saying so as a 15.57 year old.
Treating Duncan's age as a constant gets you an answer that makes a lot more sense.
Let x equal my age
Let y equal Duncan's age
current age: x = (1/9)y + 11
future age: x + 10 = (1/2)y
Solve for x and set both equations equal to each other
(1/2)y - 10 = (1/9)y + 11
(7/18)y = 21
21 divided by 7/18 = 54
y = 54
Plug 54 back into one of the original equations
x = (1/9)y + 11
x = (1/9)(54) + 11
x = 17