\(x+\left(x+1\right)+\left(x+2\right)+...+\left(x+30\right)=620\)
\(\Rightarrow\left(x+x+x+...+x\right)+\left(1+2+...+30\right)=620\)
Số các chữ số x là: \(\dfrac{30-0}{1}+1=31\left(số\right)\)
\(\Rightarrow31x+\left(30+1\right)\left(\dfrac{30-1}{1}+1\right)=620\)
\(\Rightarrow31x+930=620\Rightarrow31x=-310\Rightarrow x=-10\)