Two-Digit Numbers
Write the two digit numbers next to each other as so: 97 x 93 Note: This example is for two-digit numbers that start with the same number and have second digits that equal 10 when added together (in this example, both numbers start with 9 and the second digits, 7 and 3, have a sum of 10).
First, we multiply the two second digits together. In this case that would be: 7 x 3 = 21
Place this result to the right-hand side of the final answer. Here you can see that the final answer will look like xx21
Now add 1 to the first digit of the first number: 9 + 1 = 10
Multiply 10 by the first digit of the second number: 10 x 9 = 90
Place this result on the left-hand side of the final answer, and you'll see that you have quickly calculated the correct answer to the original problem. 9021
Two-Digit Numbers Alternate Method
Take another set of two-digit numbers to be multiplied. Keep in mind that the first digits are the same and the sum of the second digits equals 10. 98 x 92
Above each number, right the difference, or how deficient each of the numbers is from 100. 98 is -2 from 100, so write -2 above 98 92 is -8 from 100, so write -8 above 92
Cross-subtract these numbers from the value on the other side of the multiplication sign. You will see that it results in the same number. 98 - 8 = 90 92 - 2 = 90
Place this number to the left-hand side of the final answer Now you can see that the final answer will look like 90xx
Multiply the two differences together. -2 x -8 = 16
Place this number to the right-hand side of the final answer, and again see that you have quickly calculated the correct answer to the original problem. 9016
Three Digit Numbers
Take the two three-digit numbers to be multiplied and write as so: 104 x 103
Now that we are above 100, right down how much higher each number is from 100. 104 is +4 from 100, so write +4 above 104 103 is +3 from 100, so write +3 above 103
Cross-add these numbers from the value on the other side of the multiplication sign. You will see that it results in the same number. 104 + 3 = 107 103 + 4 = 107
Place this number to the left-hand side of the final answer. Now you can see that the final answer will look like 107xx
Multiply the two differences together. 4 x 3 = 12
Place this number to the right-hand side of the final answer, and even here you'll be able see that you have quickly calculated the correct answer. 10712
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