TIPS AND TRICKS FOR SPEEDY CALCULATIONS – MODULE III – MULTIPLICATION
In this module we deal with techniques using which we can multiply two numbers in an unorthodox but quick manner. Firstly we take up some specific cases using which we come to generalized multiplication of any two given numbers.
Multiplying two numbers starting with same digit(s)
Assume that there are two invisible 0 (zeroes), one in front and one behind the number to be multiplied with 11. Take an example of 234, assume it to be 0 2 3 4 0
Start from the right, add the two adjacent Zeros and keep on moving left 02340
Add the last zero to the digit in the ones column (4), and write the answer below the ones column. Then add 4 with digit on the left i.e. 3. Next add 3 with 2. Next 2 with 0.
0+4 = 4
4+3 = 7
3+2 = 5
2+0 = 2
So answer is 2574
Further examples;
36 x 11 = 0+3 | 3+6 | 6+0 = 396
74 x 11 =0+ 7 | 7+4 | 4+0 = 7 | 11 | 4 = 814 (1 of 11 is carried over and added to next digit, so 7+1 = 8 )
To multiply a two-digit number by 111, add the two digits and if the sum is a single digit, write this digit TWO TIMES in between the original digits of the number. Some examples:
36×111= 3996 54×111= 5994
The same idea works if the sum of the two digits is not a single digit, but you should write down the last digit of the sum twice, but remember to carry if needed. So
57×111= 6327 because 5+7=12, but then you have to carry the one twice.
The process is a bit complicated and shall be explained by means of an example;
Multiply 5392 by 21.
The first digit of the answer will be equal to twice the first digit of 5392. To make the rule consistent assume there is a zero before the number.
So it looks like 05392
0 + (5 x 2) = 10
Next, add the first digit of the given number, 5, to twice the second digit, 3.
5 + (2 x 3) = 11
Since we must have a single digit at each step, the tens place of the result above will be carried over and added to the previous number.
1 | (0 +1) | 1 = 111
The first 3 digits up to this point are 111
The next digit is obtained by adding 3 to twice of 9
3 + (2 x 9) = 21
Thus the first four digits of the answer are –
1 | 1 | (1 + 2) | 1 = 1131 (carried over 2 added to the last digit of 111 )
The next digit is obtained by adding 9 to twice of 2
9 + (2 x 2) = 13
Thus the first five digits of the answer are –
1 | 1 | 3 | (1+1) | 3
The last digit of the answer will be same as the last digit of the number itself.
Hence, in this case last digit will be 2.
Therefore the answer is 113232
Multiply any two digit numbers with 10 being the sum of their unit places
Principle: You will get the answer in two parts. First part, to get left hand side of the answer: multiply the left most digit(s) by its successor. Second part, to get right hand side of the answer: multiply the right most digits of both the numbers.
Multiply 45 and 46;
First part: 4 x (4+1)
Second part: (4 x 6)
Combined effect: (4 x 5) | (4 x 6) = 2024
37 x 33 = (3 x (3+1)) | (7 x 3) = (3 x 4) | (7 x 3) = 1221
11 x 19 = (1 x (1+1)) | (1 x 9) = (1 x 2) | (1 x 9) = 209
Multiply any three digit numbers with 10 being the sum of their unit places
The technique as discussed above can be extended to three digit numbers also. Tis will be made clear by solving certain examples;
Multiply 292 and 208;
Here 92 + 08 = 100, L.H.S portion is same i.e. 2
292 x 208 = (2 x 3) x 10 | 92 x 8 (Note: if 3 digit numbers are multiplied, L.H.S has to be multiplied by 10)
60 | 736 (for 100 raise the L.H.S. product by 0) = 60736.
848 X 852
Here 48 + 52 = 100,
L.H.S portion is 8 and its next number is 9.
848 x 852 = 8 x 9 x 10 | 48 x 52 (Note: For 48 x 52, use methods shown above)
720 | 2496
= 722496.
[L.H.S product is to be multiplied by 10 and 2 to be carried over because the base is 100].
C) 693 x 607 = 6 x 7 x 10 | 93 x 7 = 420 / 651 = 420651
Multiply two numbers close to 100 but greater than 100.
Principle: You will get the answer in two parts. First part, to get left hand side of the answer we add the difference between 100 and either of the numbers to the other number. For the Second part, we multiply the difference from 100 of both the numbers. Consider the following examples;
103 x 104 = 10712
The answer is in two parts: 107 and 12,
107 is just 103 + 4 (or 104 + 3),and 12 is just 3 x 4.
107 x 106 = 11342
First part; 107 + 6 = 113 and Second part; 7 x 6 = 42
123 x 103 = 12669
(123 + 3) | (23 x 3) = 126 | 69 =12669
If the multiplication of the offsets is more than 100 then this methodwon’t work. For example 123 x 105. Here offsets are 23 and 5. Multiplication of 23 and 5 is 115 which are more than 100.So this method won’t work. But it can still work with a little modification. Consider the following examples:
122 x 123 = 15006
Step 1: 22 x 23 = 506 (as done earlier)
Step 2: 122 + 23 (as done earlier)
Step 3: Add the 5 (digit at 100s place) of 506 to step 2
123 x 105 (Different representation but same method)
123 + 5 = 128
23 x 5 = 115
128 | 115 = 12915
Multiply two numbers close to 100 but less than 100.
Principle: You will get the answer in two parts. First part, to get left hand side of the answer: Add the difference between 100 and either of the numbers to the other number. Second part, to get right hand side of the answer: multiply the difference from 100 of both the numbers. Consider the following examples;
Multiply 93 and 94;
First part: 93 – 100 = – 7; Add this to the other number, thus 94 + (- 7) = 87
Or you can start with the other number 94;
94 – 100 = – 6; Add this to the other number, thus 93 + (- 6) = 87
Result will be same in both the cases
Second part:
Multiply the difference from 100 of both the numbers.
Hence, (93 – 100) x (94 – 100) = -7 x -6 = 42
Combined effect: 87 | 42 = 8742
Multiply 92 and 86;
Step 1: 92 + (86 – 100) = 78
Step 2: (92 – 100) x (86 – 100) = -8 x -14 = 112
Combined effect will look like this: 78 | 112
Step 3: Add the 1 (digit at 100s place) of 112 to 78
Answer: 78 + 1 | 12 = 79 | 12 = 7912
Multiply two numbers close to 100 one being less and the other more than 100
Principle is same as given above so we directly take up some examples;
Multiply 96 and 103;
First part: 96 – 100 = – 4; Add this to the other number, thus 103 + (- 4) = 99
Or you can start with the other number 103;
103 – 100 = 3; Add this to the other number, thus 96 + 3 = 99
Result will be same in both the cases
Second part:
Multiply the difference from 100 of both the numbers.
Hence, (96 – 100) x (103 – 100) = -4 x 3 = – 12
Combined effect: 99 | -12 = 8742
Now to remove negative sign from the right side, we have to take one from the left hand side. 1 when shifted from left to right becomes 100. Thus we’ll have:
Combined effect: 99 – 1 | 100 – 12 = 9888
Multiply 89 and 113;
= 89 + 13 | -11 x 13
= 102 | -143
In this case, right side number is greater than 100, so we need to subtract it from next higher 100, i.e. 200. Hence, we’ve to take 2 from left hand side, so that we get 200 on the right hand side.
= 102 – 2 | 200 – 143 = 100 | 57 = 10057
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