In a quiz competition, Tom is ranked 10th from the top and after swapping places with another participant, he is now ranked 15th from the top. How many participants are there in total?

B. 21

In a class of 30 students, A is ranked 10th from the top. What is A's rank from the bottom?

B. 21ST

B is ranked 5th from the bottom in a group of 25 people. What is B's rank from the top?

B. 20TH

In the sequence 100, 200, 300, 400, 500, how many times does the number 600 occur?

B. 0

In the sequence 5, 10, 15, 20, 25, 30, how many times does the number 25 appear?

A. 1