Sometimes, our ability to perform mental math diminishes over time once we become reliant on calculators for arithmetic problems. We may find ourselves doing simple multiplication problems on the calculator which we could easily do in our heads (for example, 16 times 3). Here are a few quick tools to solve these multiplication problems in your head, which will also improve your mental dexterity, and ability to think quickly.
For a two-digit times a one-digit number, (e.g. 48 times 6) you will want to multiply the 8 times 6 in your head, but only pay attention to the tens place. In this case, 8 times 6 is 48, but you should only keep the 4 in your head. Add that 4 to 4 times 6 (which is 28), and write that down. Then write down the ones place of the initial multiplication (the 8 in 48). The answer is 288. You could just write down the 8 first, but I have found many people straining to keep the carried digit (4) in their head as they write down the answer. With practice, you can perform any of these multiplications very easily.
For any two-digit number times a two-digit number, you can use FOIL. Most people use FOIL when multiplying factors of polynomials; however, this can be used for any type of multiplication. Here’s an example: 27 times 36. To FOIL this, start with multiplying the Last two digits: 7 times 6 is 42. Carry the 4 in your head, and write down the 2. Then, do Inside plus Outside (7 times 3 plus 2 times 6), which is 33. Make sure to add the 4 you carried, making it 37. Write down the 7, and carry the 3. Finally, do Forward: 2 times 3, which is 6. Add the 3 you carried, making the last digit 9. The answer is: 972. This may sound complicated, but becomes very second nature after practicing this method a few times.
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