[tex]\overline{y} [/tex] (prounounced "y bar") is the mean of the y values
Add up the y values to get
5+7+10+15+18 = 55
Then divide by 5 (because there are five y values) to get
55/5 = 11
Therefore,
[tex]\overline{y} = 11[/tex]
The answer is choice C
