9. Trick - Yet another multiplication trick

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!!!