1.1. Diligent Masha for each $n$ from 1 to 2010 calculates the sum of the first $n$ even numbers and writes it on the board. Mischievous Petya, after Masha writes down each number, erases all its digits except the last one. What will be the sum of the numbers on the board?
1.1. Answer: 4020. Note that Masha, in cooperation with Petya, for a number $n$ will write the last digit of the number $n(n+1)$ on the board. These last digits, as can be easily verified by enumeration, are periodic with a period of 5. The period has the form $2,6,2,0$, 0.