Most of us have learned by heart the multiplication table up to 10 x 10. A simple technique will enable us to extend our multiplication power up to 20 x 20.

Example 17.1:     Compute 14 x 12

• Cover one of the ten’s digit and add what remains to the other number: 14 + 2 or 4 + 12 will both give 16. This will be the first or left hand part of the product.
• Cover both ten’s digit and multiply what remains: 2 x 4 = 8. This will be the second or right hand part of the answer.
• Thus 14 x 12 = 16 | 8 or 168

What we actually did in the first step is to add the excess over the base (10) of a number to the other.In the second step we multiplied the excesses. The algebraic proof of this method, known as base multiplication, is shown below: (x + a) * (x + b) = x2 + (a + b) * x + a * b = (x + a + b) * x + a * b = [ (x + a) + b] * x + a * b

In our example above, x = 10, a = 4 and b = 2. So we have
14 x 12
= [  (10 + 4) + 2 ] * 10 + 4 * 2
= 160 + 8
= 168   

In this case both multiplicands are above the base.

Example 17.2:     Compute 16 x 13

(10 + 6) x (10 + 3)

• Add 3 (excess of 13 over 10) to 16 to get 19
  = ( 10 + 6) + 3 |
  = 19 |
  
• Multiply the excesses 6 x 3 = 18.
  = 19 | 6 x 3
  = 19 | 18
• Since there is only one zero in the base, only one place is allotted for the right hand part. Therefore the 1 of 18 must be carried or added to 19 to get a final answer of 208
  = 20 | 8 = 208

Example 17.3:     Find 107 x 104

• The left part is 107 + 4 = 111, and the right part is 7 * 4 = 28
  (100 + 7) * (100 + 4)
  = 107 + 4 | 7 x 4
  = 111 | 28

= 11,128Note that in this example we did not write the two zeroes after 111 but two places are reserved for the 28.

Example 17.4:     Find 1,025 x 1,012

• The left part is (1025 + 12 ) = 1037 and the right part is 25 * 12 = 300
  1,025 x 1,012
  = (1000 + 25) * (1000 + 12)
  = 1025 + 12 | 25 * 12
   = 1037 | 300
   = 1,037,300

Example 17.5:     Find 115 x 111

• The left part is (115 + 11) and the right part 165
  115 x 111
  = (100 + 15) (100 + 11)
  = 115 + 11 | 15 * 11
  = 126 | 165
• Since we have only two zeroes in our base, we can allot only two spaces for the right hand part of the answer, so we must “carry” the 1 of 165 into the left side.
  = 12,765

Example 17.6:     Find 102 x 104

• This is pretty straightforward with the left 102 + 4 = 106 and the right is 2 * 4 = 8
  102 * 104
  = (100 + 2) (100 + 4)
  = 102 + 4 | 2(4)
  = 106 | 08
• In this case, the product of the excesses is 8 but since the base is 100, two spaces are allotted to it. We write it as 08.
  = 10,608

Example 17.7:     103 x 119

• The left side is 103 + 19 = 3 + 119 = 122 and the right side is 3 * 19 = 57
  (100 + 3) * (100 + 19)
  = 119 + 3 | 3 * 19
  = 12,257
• Note that in this example we chose to add the smaller excess to the other number. This always leads to a simpler calculation.

Exercise 17: Find the following products using base multiplication

1. 12 x 13 =
2. 14 x 17 =
3. 15 x 18 =
4. 108 x 101=
5. 116 x 102 =
6. 108 x 112 =
7. 112 x 114=
8. 123 x 106 =
9. 1021 x 1006 =
10. 1432 x 1002 =

Answers to all exercises are found in the answer key.

Discover the 25 Math Short Cuts ( 25 MSC )!