Hello, I was given this math problem as a challenge from a friend. He apparently got it right, and my answer was wrong. Apparently mine was way wrong. Can someone explain it to me.

Country “A” (population = 100 million) has an average age of 15, and the average number of children per woman is 2.1. Country “B” (population = 100 million) has an average age of 30, and the average number of children per woman is 2.4. Which country will have the larger population growth? Explain your reasoning?

I feel like it is missing something.