Which could be the missing data item for the given set of data if the median of the complete data set is 15? 11, 23, 12, 18, 11, 10, 19, 15, 19, 21, 13, 17, 24, 14

Any number below 15, because the median (or very middle number) of this data set is between 15 and 17, so to make the median 15 one would need to add a number below 15 to counter  17, so that there would be an equal amount of numbers on either side of 15.

N- 3 is the answer