7 math tricks that’ll make your life easier

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The worst part of going out to dinner with a group of friends is figuring out who owes what at the end of the night. All too often, once everyone’s chipped in, there’s either not enough money on the table, or way too much.

When trying to plan for retirement, or figure out whether to make an investment, one helpful thing to know is how long it will take to double your initial capital.

And when you’re considering taking a new job with an annual salary, it might be useful to know how that salary compares with what you’d earn at a job with an hourly pay rate.

Some handy math tricks can help you figure out each of these situations quickly — and without having to resort to pulling out the calculator on your phone. We’ve assembled seven such tricks and talk a little about how they all work.

Eric Platt and Walt Hickey contributed to earlier versions of this article.