The moment you add three more students, the first girl is no longer one girl out of a group of three students, she is one girl (out of the two girls) out of a g

The moment you add three more students, the first girl is no longer one girl out of a group of three students, she is one girl (out of the two girls) out of a group of six students. Same with the second girl. So, once you add 3 more students, 1/3 becomes 1/6. 1/6 + /6 = 1/3.