Identify which number is being subtracted from which. If the problem is written linearly, the number to the right of the minus sign is being subtracted from the other number. For example, in the problem 43-15, fifteen is being subtracted from forty-three.
Arrange the problem vertically, if necessary. Carrying will be easier if you put the number on the right of a horizontal statement below the number on the left. After you arrange a problem vertically, a line should be placed under the problem. This will separate the problem from the solution.
Recognize the columns formed in Step 2. After you arrange the problem vertically, the example should have the 3 directly above the 5 and the 4 above the 1 -- the columns should be aligned, with the units column on the right. Separating the problem into columns will make smaller, easier problems out of the bigger subtraction problem.
Solve the smaller problems from right to left. You have separated the example problem into two separate problems of 3 minus 5 and 4 minus 1. Since 3-5 is positioned to the right, you should solve it first. This may seem complicated. In subtraction, the smaller number is supposed to be being subtracted from, not having something subtracted from it if you want to end up with a positive number. This is where carrying becomes a handy tool to have.
Borrow a "1." Look to the smaller problem directly to the left. Take the number on top and subtract 1 from it. 4-1 becomes 3-1. What happens to the 1 you just borrowed? It goes to the small problem in the units column with the seemingly mismatched numbers. 3-5 becomes 13-5. The reason it is not 4-5 is because the small problem on the left represents the "tens" column. Therefore, every 1 borrowed from that problem is actually ten. The borrowed ten is added to the top number of the problem on the right.
Solve the smaller problems. The problems now have the larger numbers on top. Now just solve the problems from right to left. 13-8 becomes 5 and 3-1 becomes 2. The differences go below the line under the problem in their respective columns. This should read 43-15=25.