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!!!!
