21. In a family, there are 4 children aged 5, 8, 13, and 15, and their names are Tanya, Yura, Svetlana, and Lena. How old is each of them, if one girl attends a kindergarten, Tanya is older than Yura, and the sum of Tanya's and Svetlana's ages is divisible by 3?
21. The girl who attends the kindergarten is the one who is the youngest, i.e., 5 years old. Tanya cannot be 5 years old, as she is older than Yura. She cannot be 15 years old either, because 15, when added to any of the given numbers, does not divide by 3. She also cannot be 8 years old, since 5 years old is not for Yura (not for a boy). Therefore, Tanya is 13 years old, then Yura is 8 years old. Then Svetlana is 5 years old, and Lena is 15.