Question 201804
Let  f = father's age 10  yrs  ago,  and s= son's age  10  yrs  ago
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f = 2s +35,,,,,we  can  continue  to  use  variable  symbols,  but  it  is  much  clearer to  use  arbitrary  numbers.

Assume  son  is  10 at  this point,,f= 2(10) +35 =55,,,all  10  yrs  ago
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Adding  10  to  both,  today  they  are  f=65,,,s=20
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father  will  be  double  if,,, (65+x) = 2(20+x)
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65+x = 40 +2x
25 = x,,,,,,,,,that  is  25  yrs  hence, father  is  twice  age  of  son
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checking  f= 65 + 25 =90,,,,,son  = 20 +25 =45,,,90/45=2,,,ok
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father  will  be  tripled  if ,,,(65+y) = 3 (20 +y)
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65+y =60 +3y
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5= 2y
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2.5 = y,,,,or  2.5  yrs  hence  father  will  be  triple  sons  age
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checking  65 + 2.5 =67.5,,,20+2.5 =22.5,,,,67.5/22.5 =3,,,,ok