**Doing Simple Stuff in Your Head Makes You Look Smart**

Want to multiply a big number by 5? Cut the number in half, and then add a zero. For instance, 88 x 5? Cut it in half to make 44, and then add a zero to make 440. (That works because you’re dividing by 2, and then multiplying by 10 — in other words, multiplying by 10/2, which is 5).

Want to divide a big number by 5? Move the decimal to the left one place and then double the number. For instance, to divide 640 by 5, move the decimal to get 64, then double it to get 128. (That works because you’re dividing by 10 and then multiplying by 2 — in other words, multiplying by 2/10, which is the same as multiplying by 1/5 or dividing by 5).

When I was in Sweden, I did a trick like this to deal with foreign currency. At the time, $1 = 7.5 kronor, which meant that you had to divide all the Swedish prices by 7.5, which is annoying. I figured out that if you move the decimal to the left one place (thus dividing by 10), and then add about a third of the number back, you get a good estimate. Later, I sat down and figured out that this actually gives you the exact number (dividing by 7.5 is the same as multiplying by 1/7.5 or 10/75, which reduces to 2/15. Dividing by 10 and then adding a third back is the same as multiplying by 1/10 and then 4/3, which is 4/30, or 2/15). Fun! People who do math in their heads are almost never doing the inefficient stuff we were all taught in middle school.

[For many more similar tricks, try Arithmetricks: 50 Easy Ways to Add, Subtract, Multiply, and Divide Without a Calculator.]

**Expected Value**

Should you buy a lottery ticket? If there’s a 1 in 10,000,000 chance of winning, and the prize is $10,000,000, then your expected value is $1. Meaning, it’s not a great deal, but certainly not the worst lottery out there.

You calculate expected value by taking the probability of winning times the prize — in this case, that’s 1. In the case of a $5 raffle ticket for a 1/1000 shot at a $10,000 car, you’d multiply 1/1000 x $10,000 to get a $10 expected value, so that’s a pretty good deal for a $5 ticket. Of course, expected value is more useful over time — for instance, if you took chances like this regularly over many years, you’d expect a profit.

**Your Value to Your Company**

If you’re a reasonable success as an entrepreneur or a freelancer, you probably do a good bit of math already. If you work at a company, you just get paid and then go home, right? And later you ask for a raise, for doing a good job?

Yes and no. A lot of “doing a good job” involves things that seemed great when you did them, but are easily forgotten: projects eclipsed by new projects, acts of productivity and ingenuity experienced by your coworkers but not so much by your superiors. You know what isn’t like that at all? Money on the spreadsheet that your boss probably looks at all the damn time.

Assume that, somewhere, there’s an Excel file with a line for your name, and then your salary with a minus in front of it, and then the cash value you have to the company.

You’ll probably never know exactly what’s on the plus-side of this spreadsheet, if it even exists, but you can calculate a good facsimile of it. Bring in new clients even when it isn’t your job. Use your networking time and your online platform, if you have one, to provide what amounts to free advertising. Pick projects most related to the bottom line (no party-planning committee). At my regular gig, I’ve written a lot of textbook chapters and done a lot of other really useful tasks that are part of larger tasks that, in sum, are important to providing the product that people pay the money for. But I also make sure to actually teach enough classes that it’s pretty clear that I was the teacher of 252 paying students this year-to-date. My spreadsheet number is black.**

If you’ve read this far, congratulations! You are not a math-phobe! Ridiculous girl-stereotypes shall not hold you back! Or, if you enjoy the girl stereotype that learning is more fun when quizzes and boy talk are involved, check out Danica McKellar’s Hot X: Algebra Exposed!

Footnotes:

*Incidentally, there is a theory about IQ distribution that could be applied to these findings as well: the idea that male intelligence is more spread out over the bell curve, and women’s is more clustered in the middle. That is, there are more male “true geniuses” and also more male total freaking idiots. Hence, the Nobel prizes on the one hand, and the prison population on the other. There are all kinds of problems and questions here well beyond the scope of this article, but even if that theory is true, the implication for what I’ve said above is actually pretty positive: if women are clustered towards the middle of the bell curve, then you’re pretty certain to have general competence at getting actual, real-world, useful things done (ideally, with panache!)

**There is a famous story — somewhat apocryphal — about an overzealous newsroom editor changing the phrase “back in the black” (as opposed to “in the red”) to “back in the African-American.”