= 27d567bc2d48d9f40e9ccc20ea370b15)
robot-dreams comments on Lifehack Ideas January 2015 - Less Wrong
You are viewing a comment permalink. View the original post to see all comments and the full post content.
= 783df68a0f980790206b9ea87794c5b6)
Loading…= b715d02e85f7b3b4ae65ca3d3e450868)
Subscribe to RSS Feed
Powered by Reddit
= f037147d6e6c911a85753b9abdedda8d)
You are viewing a comment permalink. View the original post to see all comments and the full post content.
Comments (43)
Here's how I calculate 15% tips in my head. After getting used to this approach, I could consistently beat friends who were fumbling with their smartphones.
BTW, math majors have a reputation for NOT being able to do mental arithmetic, so being a math major who could do this was a fun and ironic way to defy stereotypes.
Preparation. Memorize the product of each digit with 1.5
Step 1. Round your bill to two significant digits (35.76 -> 36)
Step 2. Recall the product of the first digit with 1.5 (3 x 1.5 = 4.5). The tip is going to be pretty close to the product you recalled (4.50)--you just need to adjust it upwards a bit using the second digit.
Step 3. Recall the product of the second digit with 1.5 (6 x 1.5 = 9), divide the product you recalled by 10 (9 / 10 = 0.9), and add it to the result of Step 2 (4.50 + 0.90 = 5.40) to get your final result.
Congratulations! You've just calculated your 15% tip to within an accuracy of 10 cents. If you want more speed, you can round your bill to just the first significant digit and skip Step 3--this will give you a 15% tip to within an accuracy of 1 dollar. Alternatively, if you want to impress your friends with more accuracy, you can keep more significant digits around and continue adjusting.
Is this really much easier than shifting the decimal place and then adding half the number? (Rounding at the start if you want, which you probably do.)
Haha, that's what I do.
If my cost is $14.32, I know $1.43 is 10%, and half of that is about $0.71, so the tip's $2.14 (though I tip 20%, which is even easier).
Right.
In the UK, we have a sales tax called VAT (for "value-added tax"). For a while its rate was 17.5%. The way you work that out is: shift the decimal point (10%), halve (5%), halve again (2.5%), and add up :-).
(Tips in the UK are usually about 10%, so that's a bit easier. And now our VAT rate is 20%.)
To expand on this method.
Take a number like 1230 10% of that number is 123.0 (found by shifting a decimal place) half of that is 61.5 add that to 123.0 =184.5 = 15%
Proposal: give waiters humanly decent salaries so they don't need the tips in the first place.
I'm strongly in favour of this but there isn't much I can do to make it happen. I imagine robot-dreams is in the same boat.