Question 543793
John is 8 years older than his sister. In 3 years he will be twice as old as she is. How old are they now?
Let s represent the age of John's sister now
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Equations:
j = s + 8
j+3 = 2(s+8)
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Substitute for "j" and solve for "s":
s+8+3 = 2s+16
s+11 = 2s+16
s = -5 (Sis's age now) 
Comment: That's the answer but that doesn't make any sense.
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Cheers,
Stan H.