Find x if 5,9,11,12,13,14,17 and x have a mean of 12.

Sum of observation = 5+9+11+12+13+14+17+x = 81+x
number of observation = 8
arithematic mean = 12

arithematic mean = sum of observation /  number of observation

so, 12 = 81+x / 8 
here we have to do the cross muliplication so 12*8=96
⇒ 96=81+x
⇒96-81 = x
⇒15= x
 so, x = 15

HOPE THIS HELPS