Question 124762
Mike is three times as old as his little sister, Mary. In 4 years, he will be twice as old as Mary. How old is Mary? 
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NOW DATA:
Let Mike's age be "3x" yrs. 
Mary's age is "x" yrs.
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4 years from NOW DATA:
Mike's age will be "3x+4" yrs.
Mary's age will be "x+4" yrs.
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EQUATION based on 4 yrs from NOW:
Mike = 2(Mary)
3x+4 = 2(x+4)
x = 4 yrs. (Mary's age NOW)
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3x = 12 yrs. (Mike's age NOW)
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Cheers,
Stan H.