12. There are 10 white balls numbered from 1 to 10 arranged in a row. Now, the balls are to be painted red according to the following method: 1) Paint 2 balls; 2) The difference in the numbers of the 2 painted balls is greater than 2. How many different painting methods are there?
Analysis: Enumeration method.
If the first ball is painted No. 1, then the other ball can be painted $4 \sim 10$; if the first ball is painted No. 2, then the other ball can be painted $5 \sim 10$;
if the first ball is painted No. 3, then the other ball can be painted $6 \sim 10$; if the first ball is painted No. 4, then the other ball can be painted $7 \sim 10$;
if the first ball is painted No. 5, then the other ball can be painted $8 \sim 10$; if the first ball is painted No. 6, then the other ball can be painted $9 \sim 10$;
if the first ball is painted No. 6, then the other ball can be painted 10;
Therefore, there are $7+6+5+4+3+2+1=28$ different ways of painting.