Hi Neelu,

According to your query, the greatesh and the smallest 4-digits numbers by using any one digit twice. digit 7 is always at ones place

Digit 7 is always at one's place
Greatest - 9 9 8 7
Smallest - 1 0 0 7

Explanation:

In case of the greatest number, remember that from values available (0-9), 9 is the greatest of all. And we can repeat the digit. So a number with 99_ _ >98_ _>97_ _ etc

In case of smallest number, you cannot have a number like  0 _ _ _. So the numnber ought to be of pattern 1 _ _ _.
Now you are allowed to use digit twice. But 11_ 7 < 1007.

So the greatest is 9987 and least is 1007.

Happy to help!