Can someone explain this problem?

60 participants of a study are arranged in a line from youngest to oldest. A researcher assigns the first third to group A, the first quartile to Group B, and the second quintile to Group C. How many of the participants are in exactly two of the groups?

@siddhisn07
Group A = 20 participants
Remaining participants = 40
Each quartile will have 10 participants.
So Group B = 10
Remaining participants = 30
Each quintile will have 5 participants
So Group C = 5

A+B = 30
B+C = 15
C+A = 25