Multiplying Two Numbers with the same Tens Digit and whose
Ones Digits add up to 10
There is a neat little trick for multiplying two numbers with
the same tens digit where the ones digits add up to 10. An
example of this type of problem is 23 × 27. The tens digit is
the same, 2, and the ones digits add up to 10, 3 + 7 = 10. Use
the following steps.
- Multiply the ones digits together. Write the product on
the right and use two digits of the answer.
- Multiply the tens digit times 1 plus the tens digit.
Write the product to the left.
Example:
23 × 27 =
- Multiply 3 × 7 = 21. Write 21 to the right.
- Multiply 2 × (2 + 1) = 2 × 3 = 6. Write 6 to the left.
621.
23 × 27 = 621.
Example:
49 × 41 =
- Multiply 9 × 1 = 9. Remember to use two digits. Since 9
is only one digit in length, write 09 to the right for
the answer.
- Multiply 4 × (4 + 1) = 4 × 5 = 20. Write 20. 2009.
49 × 41 = 2009.
Advanced Trick
This trick also works when their are more than 2 digits in the
numbers. For example, to multiply 134 × 136, you conside the
“tens digit” to be 13.
Example:
134 × 136 =
- Multiply 4 × 6 = 24. Write 24 to the right.
- Multiply 13 × (13 + 1) = 13 × 14 = 182. Write 182 to
the left. 18224.
134 × 136 = 18224.
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