Question 1136899
<pre>
a = sister's age
b = Sam's age<br>

b = 3a           (1)
b+6 = 2*(a+6)   (2) <br>

Substitute 3a for b from (1), into (2):

  (3a)+6 = 2(a+6)
   3a+6 = 2a+12
    a = 6  -->  b = 18
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Ans:  Sam is 18 years old and his sister is 6 years old.

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Check:  in 6 years:  Sam will be 24, his sister will be 12,  24 = 2*12 (ok)