6 quick math tricks to save you money
For some people, the anticipation of having to do a math problem actually registers in the area of the brain usually reserved for something that causes you physical pain, according to a recent study from the University of Chicago. Basically, if you start to sweat when you think about algebra, it could be the equivalent of someone kicking you in the shin. (And that’s not the only way numbers screw you over—see How Bad Math Skills Make You Spend More.)
Luckily, you're not in grade school and nobody is going to demand you use the Pythagorean Theorem any time soon--we're still not entirely sure what you use it for anyway--but that doesn't mean all math is useless. Here are six easy, must-have math tricks for everyday life.
1. Leave a Tip
The dinner bill comes, and no one's sure how much to add on for tip. Do this: If you want to leave a 15 percent tip, divide the bill ($36) by 10 by moving the decimal point one slot to the left. ($36.00/10=$3.60) That's a 10 percent tip, so if you add half of that ($1.80) to make a 15 percent tip you would leave $5.40, says Steven Strogatz, math professor at Cornell University and author of The Joy of X. Want to tip 20 percent? Just double the bill ($36 x 2 = 72) and move the decimal point one place for a tip of $7.20.
2. Paint a Room
You're all set to paint your dining room (10' x 10' x 12') until the guy at Lowe's asks you how many gallons of Concert Hall Red you want. First step: don't panic. Next step: Start by adding up the length of the four walls, says David Orrell, a mathematician and author of Truth or Beauty: Science and the Quest for Order. If your room is 10' x 10', that means 10 feet + 10 feet + 10 feet + 10 feet, which equals a total surface area of 40 feet. Then, multiply by the height: 40 feet x 12 feet = 480 square feet.
Next, subtract any windows or doors. (Estimate about 20 square feet per window or door.) For two windows and one door, that means 480 square feet - 60 square feet = 420 square feet. Of course, you're going to want two coats for this bad boy, so double that to 840 square feet. One gallon of paint covers 350 square feet, so 840 square feet/350 square feet = 2.4 gallons. Always round up when you paint to allow for mistakes and waste. Go ahead and request 3 gallons of that perfect color.
(Then attack these 10 Easy Home Renovations That Will Make You Rich.)
3. Convert from Celsius to Fahrenheit
You're on your Euro vacation and you're trying to figure out what to wear, but can't decipher Celsius temps. Just remember this: 50 degrees Fahrenheit is 10 degrees Celsius. After that, know that every 5-degree increase for Europeans is a 10-degree increase for us, says Strogatz. So, if it's 15 degrees Celsius outside, you're fine in shorts, since that's about 65 degrees Fahrenheit. But if the weatherman says 0 degrees? Buy a jacket--that's roughly 30 degrees Fahrenheit here.
4. Estimate the Exchange Rate
So you've found a fancy leather jacket in Florence, but you're not sure if you're getting a good deal or just being swindled by a fast-talking Italian. That's when knowledge of the exchange rate comes in handy. This one requires you to know the conversion (1 American dollar equals about .772 euro), says Orrell. Estimate how much it would cost in the U.S. ($50) and multiply that by the euro exchange (50 x .772 = 38.65 euro). If the jacket costs more than 38.65 euro, it's cheaper to buy it in the states.
5. Fetch Your Kid's Cake
You wife called to ask you to pick up the cake for your son's birthday party, so how much should you buy? If you're eyeing an Elmo cake, figure out what the serving size is--it should be on the box. If you invited 23 guests, just divide the guests by the cake serving (23/7 = 3.29 cakes), says Orrell. You'll want a healthy margin of error on this one--we hear kids (OK, and adults) cry when they don't get any cake.
6. Get the Better Supermarket Deal
If you're choosing between two discounts at the store, focus on the unit prices of each item, which will tell you how much things cost per unit of measure. To work out the unit price, you might need to whip out your cell phone, says Orrell. If a 1-pound container of ground meat is $10 but the small (1/4 pound) is on sale for $2.80, you need to divide the price by the pound to put the smaller package in the same unit of measure as the large one. For this example, $2.80/.25 = 11.20/per pound. If it only costs $10 for 1 pound, it's cheaper to go bulk.
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