Question

find the value of x is the mean of 17, 12, 19, 20, X, 21 and 22 is 18

Answer 1

sum of all terms / no of terms =mean
so
17+12+19+20+21+22+x/7=18
so
111+x/7=18
111+x=126
x=126-111=15

Answer 2

Mean => sum of observation / no. Of observation
18 = 17+12+19+20+x+21+22 / 7
18 = 111 + x / 7
18×7 = 111+x
126 = 111+x
x = 126 - 111 => 15