If it is known that at least 23 students were eligible for a Symposium on Chemistry and at max. 35 students can be selected, the minimum qualifying marks in Chemistry for eligibility to Symposium would lie in the range:
20-30
Since 66 students get 20 and above marks in Chemistry and out of these 21 students get 30 and above marks, therefore to select top 35 students in Chemistry, the qualifying marks should lie in the range 20-30.