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cum se afla x ? 5x+10x+15x+20x+...+100x=4200

Răspuns :

5x+10x+15x+20x+..+100x=4200
5(x+2x+3x+..+20x)=4200
5(20x*21/2)=4200
20x*21/2=840
20x*21=1680
20x=80
x=4