How many 3-digit numbers can these numbers make up? Their sum is equal to 1000.

Analysis: Make the sum of two three digits equal to 1000, the sum of hundreds of two three digits is 9, the sum of tens of digits is 9, and the sum of single digits is 10. First consider the hundred digits, which cannot be 1, because there is no sum of 1 to make 9. So the percentile can be 2 and 7 or 3. There are four groups (up and down as one group) 231261731761769 739 269 239 When the percentiles are 3 and 6, there are also four groups. 32137 1621671679 629 379 329, so there are eight groups of * * * and it is 1000, and * * * counts as16.