A four digit whole number can be found from 1000 to 9999
Lowest four-digit number: 1000 Highest four-digit number: 9999
7000 - 999 ie 6001 four digit numbers
2000.The smallest four-digit number there can be is 1000. So, add 1000 and 1000 to get 2000, then least sum you can get when adding two four-digit numbers.
900 numbers
There are 12 such numbers.
90
625 if numbers can have leading 0s, 500 otherwise.
1023
9,000
2240
There are four of each.
The numbers from 1000 to 9999 are all 4 digits, as are -1000 to -9999. Therefore there are 18,000 four-digit numbers in all.