MULTIPLICATION Tips
Tuesday, March 6, 2007
When two numbers are to be multiplied whose unit’s digits sum up to 10 and other digits apart from the unit’s digit are same.
e.g. 163 X 167 Here the unit’s digits sum up to 10 (7+3) and the other digits are same (16). The rule here is same as above for squaring. The rightmost part of the answer will be the product of the numbers in unit’s place i.e. (7 X 3) = 21. The leftmost part will be the product of the digit in the other places and it’s immediately following number, i.e. (16 X 17) = 272.
The required answer is 27221.
e.g. 149 X 141
Applying the same rule as above the rightmost part will be (9X1) = 09. (Remember the right part has to be always filled with two digits, so it is “09”). The right part will be (14 X 15) = 210. The required answer is 21009.
When two numbers close to 10, 100, 1000 etc are to be multiplied.
e.g. 106 X 104
Here the Base is taken as 100, and the surplus is 6 and 4 in the two numbers respectively. Now the rightmost part of the answer will be (6 X 4 = 24). Note that since the base has 2 zeros the rightmost part has to occupy 2 places.
The left part of the answer will be the sum of any one of the two numbers and the surplus in the other number, i.e. (106+4) OR (104+6) = 110.
The required answer is 11024
We can similarly do such multiplications with any base (10, 100, 1000...etc.)