This is a simple trick that is rarely taught in school. It is an easy way to divide any number by 5 using mental math. The number does not have to be divisible by 5, it works with any number. I use this trick if I am trying to calculate a 20% tip for a bill since 20% = 1/5.
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Step 1: General Formula
Step 2: Example
Basically, double the number and move the decimal point to the right or vice versa. If you're not good at doubling numbers, split the number up as shown in the next step.
Step 3: Extra: Doubling
Doubling large numbers in your head is easier when done this way. All you need is some well trained short term memory.