When using this group of numbers 12 7 18 15 11 9 what number

When using this group of numbers: 12, 7, 18, 15, 11, 9: what number should be added to the group to give a mean value of 12?

Let N=the number to be included in the group then what we want is the new mean to be 12.

The sum (12+7+18+15+11+9+N)/7=12 so solving for N gives

(72+N)/7 = 12 so N = 12

so the new set of numbers is 12,7,18,15,11,9,12