Took me 15 mins but the answer is:
902
901
903
+ 909
= 3615
There are 28706 such combinations. 5456 of these comprise three 2-digit numbers, 19008 comprise two 2-digit numbers and two 1-digit numbers, 4158 comprise one 2-digit number and four 1-digit numbers and 84 comprise six 1-digit numbers.
The four smallest three digit composite numbers are: 100, 102, 104, 105 with sum 411.
100, 102, 104, 105
300
Add the two greatest possible four digit numbers. 9999 + 9999
-3612
Since both of those numbers contains four digits, there are no three-digit numbers between them.
There are 28706 such combinations. 5456 of these comprise three 2-digit numbers, 19008 comprise two 2-digit numbers and two 1-digit numbers, 4158 comprise one 2-digit number and four 1-digit numbers and 84 comprise six 1-digit numbers.
889
1234
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
1348.
If the digits may be repeated, there are four choices for the first digit, four choices for the second, and four choices for the third. Thus there are 4x4x4 = 64 possible three-digit numbers with those digits. If the digits may not be repeated, there are still four choices for the first digit, but then only three remaining choices for the second, and then just two choices for the third. In this case there are only 4x3x2 = 24 possibilities.
The four smallest three digit composite numbers are: 100, 102, 104, 105 with sum 411.
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.