Dividing by .25 is the same as multiplying by four

A rapid math trick/tip that is very helpful to know is that dividing by 0.25 is the same as multiplying a number by four. What's even more useful to know is that multiplying a number by four is the same as doubling that number twice.

For example, today I needed to know what $1,600 divided by 0.25 was. While another person reached for their calculator, I was able to say "It's $6,400". I did that in less than a second by thinking "$1,600 doubled is $3,200, and $3,200 double is $6,400".

The way this works is pretty simple:

X / 0.25

is the same as

X / (1/4)

which is the same as

X * 4

which is the same as

X * 2 * 2

Multiplying a number by 0.25

By the same principle, multiplying a number by 0.25 is also the same as dividing that number by four, which is the same as taking the half of that number twice. So if someone every asks you "What is 9,000 times 0.25", just take 9,000 and divide it in half, which is 4,500, and divide that in half again, which is 2,250.

And so on ...

Of course this continues on into "What is 1/8 of 9,000?" Again, just take that last number (2,250) and divide it in half again, and you get 1,125. That's a lot easier than the usual manual process most people take.