Ok, so far we've seen multiplication of numbers near powers of 10 and a generic multiplication. In this we'd see multiplication of numbers near a multiple of 10. For example, multiplication of 37 and 39 or 42, both being near to 40 = 10*4. The key here is 4.
Here's the full calculation:
37 -3 42 +2 ---------- 39 | 6 ---------- => 39*4 | 6 => 156 | 6 => 1554
Superb! Hmm... note what I did in the second step 39 * 4. I multiplied by 4 because to get 40, I had multiplied 10 by 4. And since 10 has one zero, the value on the right side consists of only one digit.
Here's another way to do the same. But I will take another example: 47 * 49. Both are near 50 = 100 / 2. We will take 100 as base and the key is 2. Point to note here is that we are dividing 100 by 2 to get 50. Watch now...
47 -3 49 -1 ---------- 46 | 03 ---------- => 46/2 | 03 => 2303
What more do you ask for than converting multiplication to simple additions and at max a division by 2!!!
