keeping in mind that radius is half the diameter, we know this cone has a diameter of 2 inches, so it has a radius of 1 inch, kinda small really for ice-cream, but anyhow.
So to find the mean you add them all together and divide by the number of numbers there is so 11+15+19+17+23+20+17+18+21=174 and there are 10 numbers so it would be 174/10 which is 17.4 so you answer is 17.4