[tex]\displaystyle x \cdot (1+2+3+...+100)=60600 \\ \\ x \cdot \frac{100(100+1)}{2} =60600 \\ \\ x \cdot \frac{100 \cdot 101}{2} =60600 \\ \\ x \cdot \frac{10100}{2} =60600 \\ \\ x \cdot 5050=60600 \\ \\ 5050x=60600 \\ \\ x= \frac{60600}{5050} \\ \\ x= 12[/tex]
