Write out the subtraction problem you are dealing with in the vertical format, if it is not already laid out as such. For example, if your subtraction problem reads 519-238 write it as follows:
519
-238
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Start the problem from the right, working to the left. Since 9-8 is workable, do the subtraction and you are left with a 1:
519
-238
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Move to the next column of numbers. You will now have a situation in which you need to borrow, since you cannot subtract 3 from 1 and get a positive number as the result. Move to the last column and borrow a number. Your problem will now look like this:
4119
-2 3 8
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You can write the 1 above the existing number so as not to confuse yourself. The thing to remember is that you have taken a 1 from the 5 and added it to the 1 in the second column in order to perform the subtraction.
Solve the problem using the borrowing method:
4 11 9
-2 3 8
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2 8 1