Question 151380
Six years ago bob was four times as old as Mary. 
In four years he will be twice as old as Mary. 
How old are they? 
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Let Mary's age now be "m".
Let Bob's age now be "b".
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6 yrs. ago equation:
(b-6) = 4(m-6)
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4 yrs. from now equation:
b = 2m
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Substitute to solve for "m".
(2m-6) = 4(m-6)
2m-6 = 4m-24
2m = 18
m = 9 yrs old (Mary's age now)
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Since b = 2m, b = 2*9 = 18 yrs old (Bob's age now)
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Cheers,
Stan H.