20x +(1+6+11+.....+96)=990
20x + 970=990
20x=990-997
20x=20
x=1
(x+1)+(x+6)+...+(x+96)=990
\(\Leftrightarrow\)20x+(1+6+11+...+96) = 990
\(\Leftrightarrow\)20x + \(\frac{\left(1+96\right)\times\left[\left(96-1\right):5+1\right]}{2}\)= 990
\(\Leftrightarrow\)20x+970=990
\(\Leftrightarrow\)20x=990-970=20
\(\Leftrightarrow\)x=1
Vậy x= 1