Multiplication of 2 two-digit numbers where the first digit
of both the numbers are same and the last digit of the two numbers sum to 10
Example
To calculate 56 x 54
Multiply 5 by 5+1. So, 5 x 6 = 30. Write down 30.
Multiply together the
last digits: 6 x 4 = 24. Write down 24.
The product of 56 and
54 is thus 3024.
Multiplication of two numbers that differ by 2
(This trick
only works if you have memorized or can quickly calculate the squares of
numbers.
When two numbers differ by 2, their product is always the
square of the number in between these numbers minus 1.
Example
18 x 20 =( 19 x 19) -
1 = 361 - 1 = 360
25 x 27 = (26 x 26) -
1 = 676 - 1 = 675
49 x 51 = (50 x 50 )
- 1 = 2500 - 1 = 2499
Multiplication of two numbers that differ by 4
(This trick
only works if you have memorized or can quickly calculate the squares of
numbers.
If two numbers differ
by 4, then their product is the square of the number in the middle (the average
of the two numbers) minus 4.
Example
22 x 26 = (24 x 24 ) -
4 = 572
98 x 102 = ( 100 x
100) - 4 = 9996
148 x 152 = (150 x
150) - 4 = 22496
Multiplication of two numbers that differ by 6
(This trick
only works if you have memorized or can quickly calculate the squares of
numbers.
If the two numbers
differ by 6 then their product is the square of their average minus 9.
Example
10 x 16 = (13 x 13) -
9 = 160
22 x 28 = (25 x 25) -
9 = 616
