updated 5/23/2006 2:49:08 PM ET 2006-05-23T18:49:08

Every week, one of my kids asks me if math will really help a person in the “real world.” Toeing the line, my answer is always, “Of course.”

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For good measure, I tell a story from a previous life, when I had to get a score of 70 percent to pass a Master Electrician licensing exam in Virginia; my employer’s project depended on my passing the exam. It came down to a question of Ohm’s law and, believe it or not, my algebra kicked in: I was able to apply Ohm’s law and I passed the test with (you guessed it) a 70. So, yes, absolutely, math is incredibly useful in real life. But what happens when the math changes?

I’m talking about the metric system and foreign currencies, the bane of many American travelers. We have never gotten the hang of them, and we are paying for it -- sometimes literally. I’m pretty sure I overspend every time I travel because I don’t want to look like an idiot as I struggle to convert currency to a number with which I’m familiar. And how many items of clothing have I purchased on vacation because I misjudged the Celsius temperature? Over the years, I have come up with a few quick conversions that will help the math-challenged when they travel abroad.

liters x 4 = gallons
In most of the world, gasoline and other liquids are sold in liters, not gallons. To get an approximate idea of the gallon price, multiply the liter price by four. It’s not exact but it’s pretty close (“close enough for government work,” as they say in my part of the country).

If you are purchasing fuel, you will probably think this liters-to-gallons calculation is wrong. I assure you, it is correct. Gas costs a fortune outside the United States. American fuel prices have been a bargain forever and still are a bargain today, even though they are approaching \$3 a gallon (higher out West). For example, the price of gasoline in Milan is \$2.85 per liter. (Yes, I know the price would be in euros, but bear with me; we’ll get to euros in a minute.) Multiplying by four, the cost per gallon works out to be about \$11.40. Outrageous! Still, you need fuel, so pay for it, deal with it and move on. But turn off that air conditioner; you’ll get better mileage.

Celsius x 2 + 30 = Fahrenheit
When the front desk calls your room at 6 a.m. with your wake-up call, you tune to the news and hear that it’s 33 degrees Celsius. So, you bundle up. Wrong answer! 33 degrees Celsius is roughly 96 degrees Fahrenheit. I find it much easier to comprehend the difference between 60 F and 72 F than the difference between 19 C and 22 C, though the ranges are roughly the same. I just never got the hang of the metric system. My second-grade teacher told me I’d live to regret it, and she was right.

A simple calculation is to take the Celsius temperature, multiply it by two and add 30 — again, not exact, but it will be close enough to know whether you need the parka or the bikini. Note, however, that the quick calculation does not work at the extremes of the temperature scales, but if you’re traveling to Antarctica or the Sahara desert, you already know what the weather is going to be like.

(euro price x 2) - last number + original euro price = US dollars
They say that money is the root of all evil (and most divorces), and it certainly tends to be a problem for travelers. There are literally hundreds of currencies in this word and there is no universal conversion rule for all those pesos and rupees and bahts. But it is possible to wrap your mind around the euro, the currency of the European Union. Yes, I know the euro fluctuates more often than my son whines about video games, but recently one1 euro has cost about \$1.20.

The conversion hint for this euro/dollar exchange is a little tricky. You’ll start with the euro price and you’ll need to remember it. Here you go: To get the dollar value, double the euro price, drop the last digit and then add back the original number. (Strategically placed pause to catch up.) For example, you are looking at a beautiful Louis Vuitton golf-club bag on the Champs-Elysées with a price tag of 5,300 euros. So, take the 5,300 and double it (10,600), drop the last number -- a zero -- and you have 1,060; now add back the original euro price, 5,300, for a total of \$6,360. (Now I know why I don’t golf -- or shop on the Champs-Elysées.) It’s not an exact calculation, but it gets you in the ballpark. Checking in at http://www.xe.com/, a wonderful currency Web site, the exact conversion at press time was \$6,414.83, which reflects a higher exchange rate.

There are a few other quick conversions that may help you; they are not quite as accurate but they do help. A kilogram is about two pounds. A kilometer is about half a mile. Again, not entirely accurate, but it does help to know that 100 kilometers is roughly 50 miles -- especially when you are on the autobahn with three full bladders in the back seat asking “Are we there yet?”