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2014-10-30T07:03:19+02:00
( x + 9 + 2x ) / 3 = 7 <=>
 ( 3x + 9 ) / 3 = 7 <=>
 3 * ( x +  3 ) / 3 = 7 <=>
x + 3 = 7 <=>
x = 4.

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2014-10-30T07:26:55+02:00
(x+9+2x):3=7
(3x+9)=7·3
3x+9=21⇒3x=21-9
                3x=12⇒x=12:3
                            x=4
28 4 28