This problem is asking you to find the addition of all quadratic deviations of the values given from the mean, so we first need to find the mean value (normally written as the Greek letter "mu"). The mean value is simply the average of all given numbers:
![\mu=(12+18+15+12+4+23+26+7+8+10)/(10)=(135)/(10)=13.5](https://img.qammunity.org/2023/formulas/mathematics/college/k2aje9h9h28i1bt4lw309twpg3m0eafiix.png)
and then we perform all the squares of the quantities for each of the values (x in the formula below) given minus the mean (13.5):
![\Sigma(x-13.5)^2=448.5](https://img.qammunity.org/2023/formulas/mathematics/college/1t07ma6osmhictxjh1ab3m3virdmjtihd6.png)
Then the final answer is 448.5