Answer:
Mary = 24
Chau = 15
David = 48
Explanation:
The formula is
Mary + Chau + David = 87
And we know that
Chau = Mary - 9
David = Mary * 2
So when we fill this in
Mary + Mary - 9 + Mary * 2 = 87
4Mary - 9 = 87
4Mary = 96
Chau = Mary - 9 = 15
David = Mary * 2 = 48
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