Assume that we should find out the result of 12 x 15.
12 x 15 (Here we can write this 15 as 5 x 3)
= 12 x 5 x 3 (now 12 x 5 becomes 60)
= 60 x 3 (For this you just calculate 3 x 6, that is 18 and add one Zero to it. that is 180)
2. Multiplication by distribution
Assume that we should find out the result of 12 x 17
(Here we can divide this 17 as 10+7. Here, multiplying 12 with 17 is same as multiplying 12 with 10 and 7 separately and then adding the results)
3. Multiplication by "giving and taking"
12 x 47 (Here its little difficult for us to calculate the multiplication of 12 and 47 mentally. so just check for the ROUNDED number nearer to 47. Yes it is 50. so.....
= (12 x 50) - (12 x 3) = 600 - 36
If we have to multiply a number with 5, just divide the number with 2 and then multiply the result with 10. Confused? Its very simple step actually....
428 x 5 (Now just divide the number with 2)
= 428 x 1/2 = 214 (Now multiply it with 10)
= 2140 (This is our result)
What’s the logic behind this step?
Lets say the number is X.
Now we are dividing the number with 2. so here X becomes X/2.
And then we are multiplying it with 10. So it will become 10X / 2
Now cancel it with 2. so it becomes 10X / 2 = 5X = 5 multiplied by X. That’s it ;)
5. Multiplication by 10 ------------ just move the decimal point one place to the right
6. Multiplication by 50 ------ divide with 2 and then multiply by 100
Well, this is also same process as we did for 5. Here we should add an extra zero. That’s it
7. Multiplication by 100 -------- move the decimal point two places to the right
8. Multiplication by 500-------- divide with two and multiply with 1000
9. Multiplication by 25 ---------- use the analogy Rs 1 = 4 x 25 Paise
25 x 14 (just divide the 14 as 10+4)
= 250 + 100 ---> Rs2.50 + Rs1
10. Here you can use another technique too. Which we have used for multiplication with 5.
Multiplication by 25 ----------- Divide by 4 and multiply by 100
11. Multiplication by 11 (if sum of digits is less than 10)
= 7+2 =9, it is Less than 10. so,
= place this term 9 between 7 &2
= 792 (That's the answer)
12. Multiplication by 11 (if sum of digits is greater than 10)
because here 15 is greater than 10, first use 5 and then add 1 to the first term 8,
which gives you the answer
13. Multiplication of numbers ending in 5 with the same first terms (square of a number)
first term = (2 + 1) x 2 = 6
answer = 625 => square of 25
first term = (7 + 1) x 7 = 56
answer = 5625 => 75 square
