Take 10% of the bill -- move the decimal point -- which shows how many cents -- one place to the *left*.
Now take half of that number.
Add the two together.
It’s even easier in WA state - tax is 10%. I just wish my wife didn’t always have to ask me. As I said, I’d rather just plead ignorance on how much money we spent.
Actually - it’s more like:
“So the tax is $6.43 - what would the tip be?”
“Hmm - give her $13.”
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Wait while she pulls out her phone.
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“It’s $12.86. I’ll just give her $13.”