Explanation:

Some students participated in a quiz competition and got different ranks. 1st rank is considered as highest, 2nd as second highest and so on.
Find the total number of participants?
Statement I: Radha was ranked 13th from the top. Rani’s rank is 16th from the bottom. Only three participants were ranked between Rani and Suresh.
Statement II: Rani’s rank was better than Sudha, who was 12th from the bottom. Suresh’s rank was exactly between the ranks of Radha and Rani.
Checking statement I:
We can have following two cases:
I1
But we don’t know the position of Rani from top, in order to determine the total number of participants.
Hence data in statement I alone is not sufficient.
Checking statement II:
We cannot determine the total number of students as we don’t know the rank of anyone except Sudha.
Checking statements I and II:
As per the above image, total number of participants can be calculated as per the following method.
We have Suresh whose position from top and bottom is known.
Total number of participants = (Suresh’s position from bottom + Suresh’s position from top) – 1
= (20 + 17) – 1 = 36
Hence data in both the statements is necessary to answer the question.