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Let sister be s.  Matthew, M

Matthew is 7years older than twice the age of his sister

M = 7 + 2s

13 = 7 + 2s.   7 crosses the equality to other side and becomes -7

13 - 7 = 2s

6 = 2s    Divide both sides by 2

6/2 = 2s/2

3 = s

Sister is 3 years old.
The answer is a 3. because Matthew is 7 years older than twice the age of his sister. Also Matthew is 13 so the answer is A 3.

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