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Multiplication by 5

To multiply 5 times an even number, halve the number you are multiplying by and place a zero after the number.

For example the product equals 30 and here half of 6 is 3, or   and again half of 12 is 6.

The proof is trivial .

In case of an odd factor we have . This gives:

To multiply 5 times an odd number subtract one from the number you are multiplying, then halve that number and place a 5 after the resulting number.

For example , in fact , half of 10 is 5, place a 5 at the end of the resulting number to get the product 55.

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