Question 884817
I need to make some assumptions here.  If they are incorrect, please repost.
First, positive numbers.
Second, did you mean subtracting a three digit number from a four digit number?
Third, we're not using zeroes.
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What is the smallest four digit number? 1111
What is the largest three digit number? 999
1111 - 999 = 112
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If you're using zeroes, then
The smallest four digit number is 1000 (you can't use a zero in the thousands place because then it's a three digit number)
The smallest three digit number is 100.  Subtract 900 from 1000 to get 100.