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Rita is 12 years old and her father is 39 years old. In how many years will Rita be twice as young as her father?

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3 Answers

2(12 + n )= (39 + n)   (1)
 
 24 + 2n = 39 +n 
 
        n = 39 -24 = 15
 
    After 15 years Rita 's age = 12 + 15 = 27, father age = 39 +15 = 54
 
     54 = 2 ( 27).
 
  Notice that whole word problem was put into one algebraic statement of equation (1). 
 
 
Moshe,
        Another less intuitive way to do this problem….
 
Let Af=t+39, the fathers age, t is time in years (t=0 now)and Ad=t+12, the daughters age then we want to calculate at what time will Ad=Af/2 so we must have t+12=(t+39)/2, solving this equation for t gives t=15 years. In 15 years Rita will be 15+12 or 27 years old and her father will be (15+39)/2 =54 years old.
 
Jim
Subtract the two ages:
 
39 - 12 = 27
 
Rita will be half as old (twice as young) as her father when she is 27 - or in 15 years.