Question

Aflati numarul x din: 8x + 16x + 24x + 32x +...+96x =6240

Answer 1

8x + 16x + 24x + 32x +...+96x =6240
                      ⇵
8x*(1+2+3+4+ ... + 12) = 6240   | :8
           x * (12 * 13 : 2) = 780
              x *  ( 78 ) = 780   | : 78
                        x = 10 

Answer 2

$\displaystyle \mathtt{8x + 16x + 24x + 32x +...+96x =6240} \\ \\ \mathtt{8x(1+2+3+4+...+12)=6240}\\ \\ \mathtt{8x \cdot \frac{12(12+1)}{2} =6240} \\ \\\mathtt{8x \cdot \frac{12 \cdot13}{2} =6240}\\ \\ \mathtt{8x \cdot \frac{156}{2} =6240} \\ \\ \mathtt{8x \cdot 78=6240} \\ \\ \mathtt{624x=6240}\\ \\ \mathtt{x= \frac{6240}{624}} \\ \\ \mathtt{x=10}$