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 From B. Lee Clay

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Multiplying a repeating 5's number by 33

1. Choose a repeating 5's number (555, etc.)
2. Multiply it by 33
3. The product is:
- the first two digits are 18
- the next digits are repeating 3's (two fewer than repeating 5's in the original number)
- the last two digits are 15

Example:

1. If the first number is 55555:
2. Multiply by 33. The product is:
- the first two digits are 18: 18
- the next digits are repeating 3's (two fewer than repeating 5's in the original number): 333
- the last two digits are 15: 15
3. So 55555 × 33 = 1833315.

See the pattern?

1. If the first number is 55555555:
2. Multiply by 33. The product is:
- the first two digits are 18: 18
- the next digits are repeating 3's (two fewer than repeating 5's in the original number): 333333
- the last two digits are 15: 15
3. So 55555555 × 33 = 1833333315.
Remember these three steps and you will be able to give these large products with ease.