My brain has been gifted with a pretty good spell-checker but not so good calculator. Instead of doing straight out calculations, I sometimes resort to some math magic. Let’s take going out to eat at a restaurant as an example. When going with a group, it’s more complicated if people want to pay according to what they ordered. I used to resort to taking out my mobile phone or PDA to use the calculator. I haven’t had to do that since I figured the trick to calculating tax and tip without a calculator.
To start off, I don’t try to figure out tax and then tip. I calculate them together. Where I live, the tax rate is 8.25%. Tip is 15%. The sum percent of tax and tip is 23.25%. To simplify matters in my simple brain, I round up to 25%. The tip will be more than 15% but that’s ok with me.
Twenty-five is a good number because it makes it easy to deal with subtotals that are factors of 4. For example, if a meal costs $12, tax & tip (t&t) will be $12/4=$3 . The total will be $15. A $100 meal will total $125.
How about when the meal cost is not a factor of 4? No problemo. What if it is $42? This is the thought process in my head to arrive at t&t:
Double 42: 84 (200% of 42)
Divide 84 by 10, or move the decimal to the left by one: 8.4 (20% of 42)*
Divide 42 by 10, or move the decimal to the left: 4.2 (10% of 42)
Divide 4.2 in half: 2.1 (5% of 42)*
With those pairs, you can do the decimal change before or after the doubling or halving, depending on which one makes it easier to do the math.
* Add the values: 8.4+2.1 =10.5 (20%+5% = 25%)
Cost + T&T = 42 + 10.5 = $52.50. This number could be rounded off to $52 or $53
It might seem complicated but it’s not that hard. At least it’s not as difficult as it would be to try to come up with exact numbers:
Tax: 3.47 (8.25%)
Tip: 6.3 (15%)
It’s lower than the estimated cost, but it took more effort to go through, if one isn’t blessed with a built-in calculator.
I’ll try another example but with more of a stream-of-consciousness method to the walkthrough: $27
27… 54… 5.4… (20% of 27)
27… ~13… 1.3… (5% of 27)
27+ ~7= 34 total including t&t
exact sum: 33.2775
Well, I hope that helps, and doesn’t end up causing confusion.