Quickanddirtytips.com shows how two helpful tips make solving subtraction problems easier. The first shows to combine addition and subtraction strategically. The second demonstrates breaking up difficult problems in order to simplify them.
Addition and subtraction are combined by adding enough to the subtracted number to make it a round number, then adding the same amount to the answer. If the problem were 151 - 97, for example, add 3 to 97 to make 100. Then subtract 100 from 151 and add 3 to the result.
Difficult problems can be separated into simpler groups and solved. Adding the solutions yields the answer to the original problem. For example, the problem 1137 - 623 becomes 1,100 - 600 = 500, 30 - 20 = 10, and 7 - 3 = 4. Add these answers to solve the original problem: 500 + 10 + 4 = 514.