Răspuns:
Explicație pas cu pas:
(x+2)+(x+4)+(x+6)+.....+(x+20)=140 <=>
10x + (2+4+6+...+20) = 140 <=>
10x + 2·(1+2+3+...+10) = 140 <=>
10x + 2·(11+11+11+11+11) = 140 <=>
10x + 2·55 = 140 <=>
10x + 110 = 140 <=>
10x = 140-110 =>
10x = 30 =>
x = 30:10 =>
x = 3
