[tex]\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}[/tex]
