Question 4
Here is a method for squaring a number ending in 5. For $25\times25=625$, calculate $2\times(2+1)=6$ and append 25. Similarly, $35\times35=1225$ uses $3\times4=12$, and $45\times45=2025$ uses $4\times5=20$. These squares always end in 25. Find the sum of the digits of $455\times455$.
