I remember staying over at my aunt’s house during my school holidays when I was about 9 or so. Whilst I was in bed trying to sleep, I had an epiphany, and figured out how to make it easy to divide by 5. I thought I was a genius! Now, I realise how obvious it actually is – but still, it’s a useful trick for anybody. I use it (and other such shortcuts) all the time due to my terrible mental arithmetic speed!
The Quick Way to Divide by 5
Take a number which you want to divide by 5 (for example, a number which you would like to find 20% of). I’ll use 285.
Hopefully, you’ll know that a number is divisible by 5 if and only if it ends in a 0 or a 5. So we know that 285 is indeed divisible by 5.
Now, ignore the last digit for a moment, and take the number in front of it. In this case, we have 28. Double this number – 56. Finally, look at your last digit. If it is a 0, then your doubled number is your answer. If it is a 5, then add 1 to your answer – so in my case, 56 + 1 = 57 – and you can check that 285/5 = 57.
Once you have this basic method internalised, it’s also easy to find the exact answer for numbers not divisible by 5. I’ll pick a random large number, say 36574.
First, divide 36570 by 5, because we know this is divisible by 5. This comes to 7314 (3657 x 2). Now we have 4 left, so 4/5 = 0.8, and adding this on to what we had, the answer is 7314.8.
The beauty of this is that the steps are extremely simple for your brain to process, and so you can get very fast in a short amount of time.
How it Works
Not much to it, really. We are dividing the number by 10, and then doubling it, so that the original number has only been divided by 5. (When I realised what I was actually doing, I felt silly for thinking that this was so great!)
Have fun with it, and remember – maths is a tool to help make your life easier!