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Multiplication by 5, 50, 500 and 0.5

A question given in the Juniors division of the 1st International Vedic Mathematics Olympiad deals with the short method of multiplication by 5:

“684.4 × 5 = 3422 . Which operation gives a quick method for multiplying numbers by 5?

A) Halve the number and multiply by 10

B) Double the number and divide by 10

C) Halve the number and divide by ten

D) Double the number and multiply by 10

E) None of these”

The easiest way to know the answer is to express 5 as 10/2. So to multiply by 5, we can multiply first by 10 and then divide by 2(halve) or halve the number first then multiply by 10. We can see that Choice A is the correct answer.

The techniques for halving or dividing a number by 2 is discussed in https://www.math-inic.com/blog/dividing-by-2/ while multiplying by 10 is done adding a zero at the end of a number or simply moving the decimal point one place to the right.

To multiply by 50: halve the number and multiply by 100.

To multiply by 500: halve the number and multiply by 1000

To multiply by 0.5: just halve the number.

Try these multiplications.

  1. 246 x 5 =
  2. 3478 x 5 =
  3. 156358 x 5 =
  4. 682 x 50 =
  5. 3658 x 50 =
  6. 6842 x 500 =
  7. 34336 x 500 =
  8. 2648 x 0.5 =
  9. 532546 x 0.5 =
  10. 137934 x 0.5 =
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