Multiplying a 2-digit number by 396

1. Select a 2-digit number.
2. Multiply by 4.
3. If the product has 2 digits, subtract 1: X X _ _
   Subtract the product from 100: _ _ X X
4. If the product has 3 digits, add 1 to the first digit:
   X X X _ _
   Subtract this from the product;
   Subtract the last 2 digits of the product from 100:
   _ _ _ X X
5. If the product has two final zeros, subtract the 1st digit from the product: X X X _ _
   The last 2 digits are 0 0 : _ _ _ 0 0

   Example:

1. The 2-digit number chosen to multiply by 396 is 18.
2. Multiply by 4: 4 x 18 = 40 + 32 = 72
3. Subtract 1: 72 - 1 = 71: 7 1 _ _
4. 100 - product: 100 - 72 = 28: _ _ 2 8
5. So 18 x 396 = 7128.

   See the pattern?

1. The 2-digit number chosen to multiply by 396 is 68.
2. Multiply by 4: 4 x 68 = 240 + 32 = 272
3. 1st digit + 1: 2 + 1 =3
4. Subtract from product: 272 - 3 = 269: 2 6 9 _ _
5. 100 - product's last 2 digits: 100 - 72 = 28: _ _ 2 8
6. So 69 x 396 = 26928.

1. The 2-digit number chosen to multiply by 396 is 50.
2. Multiply by 4: 4 x 50 = 200
3. Subtract 1st digit: 200 - 2 = 198: 1 9 8 _ _
4. Last 2 digits are 0 0: _ _ _ 0 0
5. So 50 x 396 = 19800.

Practice multiplying by 4 from left to right. Repeat the first digits of the answer, then add the final two. Your speed with this large multiplication problem will be impressive.