MULTIPLYING TWO TWO-DIGIT NUMBERS WHOSE TENS DIGITS ARE THE SAME
Rule: Add the units digit of one number to the other number. Multiply the result by the tens digit and affix a zero to the product. Add the prod-uct of the units digits to the result.
Since multiplication by the digits is the key step in this short cut, a review of Short Cuts 8 to 15 is recommended. Example: Multiply 72 by 79.
Add the units digit of one number to the other number. It does not matter whether 72 is added to 9 or 79 is added to 1; the result is the same.
72 + 9 = 81 or 79 + 2 81 Multiply by the tens digit, 7. 81 x 7 = 567 (Short Cut 13) Affix a zero 5,670 and add the product of the units digits. 2 x 9 = 18 5,670 + 18 = 5,688
2 A 9 = 18 5,670 + 18 = 5,688
Therefore
72 x 79 = 5,688 Answer
