It is a permutation problem (any number can be used repeatedly), it will become 4 x 4 x 4 x 4 = 256 four digit numbers. If each number can only be used once, then the answer is 4 x 3 x 2 x 1 = 24 four digit numbers.
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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.
There are 4 ways to chose the first digit. Then because there is to be no repetition, there are 3 ways to choose the second. This means that there are 4 x 3 ways of setting down the first two digits 12 _ _ 13 _ _ 14 _ _ 21 _ _ 23 _ _ 24 _ _ 31 _ _ 32 _ _ 34 _ _ 41 _ _ 42 _ _ 43 _ _ .... now you can take it from there.
Add the two greatest possible four digit numbers. 9999 + 9999
8100
7