Given
mean of 15 observations = 16.4
sum of all abseravations/number of observations=16.4
(sum of all observations)/15=16.4
sum of all observations=16.5 *15 = 247.5
if 1 observation is cancelled, mean = 17
sum of all observartions=17*14=238
so the cancelled observation = 247.5 - 238 =9.5