Multiply by 5 In Your Head

mental-math

Multiplying by 5 mentally is easy since you just divide the number to be multiplied by 2 and multiply the result by 10, (also very simple, by just adding 1 zero to the number or if the number has a .5, just move one decimal place to the right).

Examples:

34 × 5

Thought Process:

1. 34 ÷ 2 = 17

2. 17 × 10 = 170

Therefore, 34 × 5 = 170

63 × 5

Thought Process:

1. 63 ÷ 2 = 31.5

2. 31.5 × 10 = 315

Therefore, 63 × 5 = 315

There are many posts about more sophisticated mental multiplication techniques coming up, though I have to cover the basics first for the sake of completeness.

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My name Edmark M. Law. I work as a freelance writer, mainly writing about science and mathematics. I am an ardent hobbyist. I like to read, solve puzzles, play chess, make origami and play basketball. In addition, I dabble in magic, particularly card magic and other sleight-of-hand type magic. I live in Hong Kong. You can find me on Twitter` and Facebook. My email is edmarklaw@learnfunfacts.com

25 thoughts on “Multiply by 5 In Your Head

      1. Interesting. Where did you learn it?

        I asked since I haven’t found a reference of this yet. I have no doubt that this method has been around for a while now but the only reason I know this is I found the theorem myself.

        Liked by 1 person

    1. From what I’ve seen from your posts, your grammar is good.

      While I’ll never accept even minor grammatical errors on academic papers, it’s not that big of a deal in other types of writings as long as you don’t commit basic errors, and your writings are not ambiguous.

      Besides, the English language is a continuously evolving language and it is mostly defined by usage instead of a fixed set of stringent and encompassing rules.

      Like

  1. My husband does it this way, but I get confused when I try it. I’ll stick with the way I was taught in grade school, my brain doesn’t handle change very well! 😉

    Liked by 1 person

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