That’s why it’s called an estimate. I’m not talking about exact figures, just a finger in the wind quick estimate. I can do the real math in my head, of course, but sometimes, I dunno, like in a lumber yard looking for the cost of 14 sheets of finished plywood at $30 a pop, it’s nice to know “Hmm, that’s about $500.” I know 3 does not equal 3.33, so I would naturally round down a bit and come up with $450.

I’m surprised how many people cant think like this. That’s all.

Grade school long division was the last math I understood. The only way I passed high school math was because the nun only taught the boys; she passed the girls so she wouldn’t look bad. My college math classes were taught by heavily accented foreign professors, who also passed everyone because no one could understand them. My semester long accounting project passed because the professor felt sorry for me. I don’t think he had ever used that much red ink on a student ledger before. Just looking at numbers makes me anxious.

I tried to explain to my teenager that math is really just a language. It’s a set of rules for manipulating things, and once you get the rules down, you can mix and change them all around to find cool ways of doing stuff like this. It never sunk in, I’m afraid.

In other news, we had a new grad student work in our lab for a while who was baffled at the thought of multiplying or dividing by ten in her head. I was showing her how to do some dilution, and I said something like “we need one tenth of 8.34, so we use .834…” She stopped me there and started writing down numbers on her paper. I must have looked a bit surprised, because she said, “Oh, I’ll just trust you for now and work it out completely later.” She didn’t last long in the program.

My method for something like that is 10x30 (300) + 4x30 (120) = 420, which is actually a bit faster for me than dividing by 3. (With the other one, I’d do 200x3 - 25x3, also a quickie.) But, yeah, point taken that the ability to estimate with more difficult problems is a good skill to have, no matter how you do it (like with three or four digit numbers, I’d start lopping off some ending digits.)

It’s *exactly *like multiplying by 2 and dividing by 10! Because 10 divided by 2 is 5. What you did was perfect, and much easier for most people to do in their heads. You did, ultimately, divide by 5, by using two operations that equal 5! It’s awesome that you figured this out intuitively.

This is exactly the sort of strategy that modern math education in the US is trying to give our students. Exactly the kind of thing that’s making parents’ heads explode on Facebook ranting about “Common Core Math”. But it makes things so much easier in the long run!

Another example: my daughter recently asked me “what’s 12 times 7?” I’ll be honest, even I don’t have that one memorized. So I asked her to come up with a strategy.

“Well, I know 12 is 6 X 2… so if I do 6X7, that’s 42, and double it is 84?”

PERFECT! She was able (with fourth grade “Common Core math,” 9 years old) to intuit that 2(6X7)=12X7. She’s “factoring” the 12, she just doesn’t have that word for it yet! I wouldn’t have been able to do that until 7th or 8th grade pre-algebra back in my day.

As for the OP: I get stuck a bit on the upper end of the 9 multiplication table. Up to 9X5, I’ve got it just by rote. After that, for years I’d struggle. 9 X 6 is, um…53? 56? Crap.

Then I learned that the 9’s have a neat pattern. Up to 9X10, the answer’s digits add to 9. So if I can remember the first number, and I can, whatever else adds to that to make 9 gives me the right answer.

As an American who used to visit Canada for work (and talk to Canadians on the phone daily), I found it useful to be able to estimate temperature conversions between F and C quickly. Classic formula:
9/5 * C + 32 = F

If all I want to know is if it’s cold or hot…would I need a bathing suit, a light jacket, or ski clothes…then 9/5 is a bit less than 2 and 32 is a bit more than 30. So
2 * c + 30 is close to F

You can’t use this for science or cooking, but it works well for casual conversation.

When I need a pretty-close-but-not perfect conversion of kilograms to pounds, I double the number and add the first two digits to the tens and ones. That’s hard to put in words.

60 kilo. Double it, 120. Add 12 to get 132 pounds. Actual conversion: 132.27

70 kilo. Double it, 140. Add 14 to get 154 pounds. Actual conversion: 154.32

85 kilo. Double it, 170. Add 17 to get 187 pounds. Actual conversion: 187.39

I haven’t figured out an easy way to go from pounds to nearly accurate kilos in my head yet, though. Anyone have one?

Doing arithmetic left to right rather than right to left, as I was taught (back in the early 80s). 48Willys already did this upthread:

Particularly useful when you don’t need an exact number, or for quickly eliminating wrong answers on a multiple choice test. Apparently left to right is taught now as part of the Common Core, but back in my time right-to-left was “the proper way.”

The [del]New Math[/del] Common Core stuff calls that the “partial products” method. I think it’s a far better way to teach it. Students still learn the traditional algorithm later, since it’s faster to do on paper for larger numbers. But the partial products method makes it a lot clearer conceptually.

Whoops, I meant 1% of the final step back in, so it should be:

220/2 = 110. Subtract 10% (11) = 99. Add 1% back in = 99.99 vs the true value of 99.79. But the two-step process should be enough for most estimations. Just know that with two steps, you’re going to be slightly under. (The two steps essentially gives you a conversion factor of 1 lb = 0.45 kg instead of 0.453592 kilograms, the true value.)

To convert Celsius to Fahrenheit for weather purposes, double it and add 30. Don’t try this with cooking or scientific experiments.

I work as a cashier in a store where every price ends in .99. If I’m posting volumes (i.e., two at), I make sure the final price ends in a number that, when the number of items is added to it, makes 0. Two items? 8. Three? 7.