We need to find out the smallest number that is a multiple of both 3 and 6. This is the least common multiple.
List the multiples of each number. Find the smallest number that appears in both lists.
Multiples of 3: 3, 6, 9, 12, 15
Multiples of 6: 6, 12, 18, 24, 30
The least common multiple of 3 and 6 is 6. That means that the smallest number of athletes each classes must have selected is 6, because 2 groups of 3 from class 6A is 6 athletes in total and 1 group of 6 from class 6B is 6 athletes in total.
The smallest number of athletes each classes must have selected is 6.