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Luna, Lily, and Lucy are talking about their ages. $6$ years ago, Luna was born, and Lily was $5$ years old. This year, the sum of ages of these $3$ people is $33$. When Luna was born, how old was Lucy? 
Overseas Competition->Knowledge Point->Word Problem Modules->Age Problems
Six years ago, the sum of ages of Luna, Lily, and Lucy was $33-6\times3=15$. Lucy was $15-0-5=10$ years old. 