It turns out, your school teacher was right: you need maths on a regular basis. While most of us won't have to calculate orthogonal trajectories on a regular basis, here are some of the most vital maths shortcuts for your everyday life.

## Use The Rule Of 72 To Estimate Interest

When you're considering where to invest your money and what kind of return you want to get, the Rule of 72 is a useful rough-and-ready tool.

*72/Interest Rate = Years To Double Investment*

The Rule of 72 isn't perfectly accurate, but it's a useful quick starting point. Say you have $5000 and you want to place it in an investment that has a return of 10 per cent. That $5000 will double to $10000 in about 7.2 years.

## Calculate A Tip In Your Head Quickly

If you're in a tipping-centric culture such as the US, you'll need to calculate how much to leave. To simplify how much you need to give for a tip, move the decimal in your total one place to the left to reach 10 per cent. If your total is $20, you get $2 for a 10 per cent tip. From there you can easily calculate a higher percentage. If you want to do a 15 per cent tip, multiply that number by 1.5 ($3 in our example). For 20 per cent, multiple by two, and so on. (15 per cent is generally considered the minimum in a US context.)

## Convert Between Fahrenheit and Celsius

Another US-centric annoyance is converting from Fahrenheit and Celsius (the 0-100 scale from freezing to boiling on the latter is massively more logical, but try telling the USA that). It's relatively easy to flip between the two however:

- Celsius to Fahrenheit: Multiply the temperature by 2, then add 30.
- Fahrenheit to Celsius: Subtract 30, then divide by 2.

If you do need to get more specific and have access to a calculator, replace 2 with 1.8 and replace 30 with 32 in the above equations.

## Figure Out The Annual Cost Of Daily Habits

We all have habits that cost small amounts of money when we indulge, but which can add up to surprising sums. If you want to find out just how much you'll spend on them every year, there's a quick way to find out:

- Estimate the weekly cost of each purchase. If it costs $4 and you do it four times per week, then the weekly cost is $16.
- Add two zeros to the end. In this example, we get $1600.
- Divide that number in half. Here, we get $800. This will be a rough approximation of the yearly cost.

This will actually give you the cost of 50 weeks of the habit, but it's reasonable to assume there would be some variation throughout the year. It's still a great way to get a frame of reference for how much you might be able to save by changing your habits.

## Comments

C to F - if you have slightly better mental arithmetic skills, don't use a calculator. Divide by 5, multiply by 9, add 32.

I was taught, what I think is an easier way,

for C to F , double the Celsius figure then take off 10% then add 32

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