Here is a method for multiplying a number ending in 5 by itself.
$25\times25=625$: $2\times(2+1)=6$, followed by $25$.
$35\times35=1225$: $3\times(3+1)=12$, followed by $25$.
$45\times45=2025$: $4\times(4+1)=20$, followed by $25$.
The result always ends in 25, since $5\times5=25$. Find the sum of the digits of $255\times255$.
