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2014-11-16T17:13:02+02:00
X+(x+1)+(x+2)+...+(x+19)=310
(x+19)+(x+18)+(x+17)+...+(x+1)+x=310
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2x+2x+20+2X+20+...+2x+20=310·2
40x+20·19=620
40x+380=620
40x=240
x=6