Question 1066924
x = child's current age
x + 12 = age in 12 years
3x - 6  = "six years less than triple her current age"

x+12 = 3x-6
x+12 + 6 -x = 3x-6 +6-x   (added 6-x to each side)
     18 = 2x
      18/2 = 2x/2
         9 = x

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Ans: the child is {{{highlight( 9 )}}} years old
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Check: in 12 years, the child will be 21
          and  3 * 9 - 6 = 27-6 = 21  (ok)