In the video “Standard deviation and other things practice” of the Quant Concept series, in the 6th question you discussed (enclosed a screenshot), I think we can’t determine the answer without an assumption.
The answer which Greg has arrived at is, in my opinion, under the assumption that all students are of different ages. If this assumption doesn’t hold:
(i) In the beginning, Frank can be anywhere between the first child in the second quartile [Eg case of 2nd quartile: 2,3,4,4,4,4,4,6] (absolute position 9) and the third last child in the quartile (absolute position 13)
(ii) After adding 12 students, Frank’s min absolute position will be 12+9=21 (i.e, quartile 2) and max would be 12+13=25 (i.e; quartile 3). So Frank can be anywhere between position 21 and 25 based on the numbers.
Please correct me if I am wrong.