Multiplying any number by a 2-digit number ending in 9

Rule: 

Multiply by 10 (append a ‘0’ after the multiplicand) and then multiply by one more than the tens digit of the multiplier (number ending in 9). After that, subtract the given number from the result.

For example, multiply 713 by 39.

First multiply 713 by 10 to get 7,130.

One more than the tens digit (3) of the multiplier is 4. Multiply 7,130 by 4 (using Short Cut doubling twice).

7,130 x 4 = 28,520

Now, subtract the given number from this finding to get the final answer.

28,520 – 713 = 27807 (Answer)

Another Example: 24,653 × 79 =?   

Multiply 24653 by 10 to get 246530.

Multiply 246530 by 8 to get (246530 x 8 =) 1972240.

Subtract the given number to get the answer (1972240 – 24653 =) 1947587.

This short cut can be applied to any number, no matter how many digits it has, so long as the unit’s digit is 9. Of course, as the number gets larger, multiplying the two numbers of the first step will become cumbersome unless a short cut can be used. However, most two- and three digit numbers ending in 9 can be readily squared, once a facility with the other short-cut methods has been achieved.