<PRE><B>Question 34250</B>
let Mary be M years old and her brother be B years old

Mary is three years older than her twin brothers, therefore
M = 3 + B -- eq1

Next year, the sum of the ages of the three will be exactly 102. 
Since each one of them will grow 1 year old, therefore,
102 = (M+1) + (B+1) + (B+1) -- eq2

Sub eq1 into eq2,
102 = (3+B+1) + (B+1) + (B+1)
102 = 3B + 6
96 = 3B
B = 32

Put B = 32 into eq1,
M = 3 + 32
M = 35

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