`N=5+5^2+5^3+5^4+...+5^2020`
`=>N=(5+5^2+5^3+5^4)+(5^5+5^6+5^7+5^8)+...+(5^2017+5^2018+5^2019+5^2020)`
`=>N=5(1+5+5^2+5^3)+5^5(1+5+5^2+5^3)+...+5^2017(1+5+5^2+5^3)`
`=>N=5(1+5+25+125)+5^5(1+5+25+125)+...+5^2017(1+5+25+125)`
`=>N=5.156+5^5 .156+...+5^2017 .156`
`=>N=156(5+5^5+...+5^2017)`
`=> N vdots 156`
`=> N` là bội của `156`