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2016-01-27T15:38:50+08:00

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12 and 9 is the answer
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2016-01-28T08:33:13+08:00

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Equation 1)    x + y = 21                                    
                             y = 21 - x   

Equation 2)  2x - y = 6

Solve the system by substitution:
Substitute  21-x for y in Equation 2:
     
         2x - (21-x) = 6
         2x + (-21 + x) = 6      (Change the sign of each term of  the subtrahend,    then add/combine like terms)

         2x + x - 21 = 6
         3x = 6 + 21
         3x/3 = 27/3
         x = 9     

Substitute 9 for x:
   First number: x = 9
   Second number: 21-9 = 12

ANSWER:  The numbers are 9 (first number) and 12 (second number).

Check:
9 + 12 = 21
21 = 21    (True)

2(9) - 12 = 6
18 - 12 = 6
6 = 6  (True)
         
 
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