Everyday Math for Everyday Life

A Handbook for When It Just Doesn't Add Up
by Mark Ryan

Math Can Be Easy!

Tackle common math problems:

• Determine the gas mileage of your car
• Calculate your daily caloric intake from FDA food labels
• Convert a recipe from eight servings to ten
• Compute the true interest rate of your credit card
• Demystify the prime rate and consumer price index.

Do mental math on the spot:

• Convert foreign currencies
• Calculate discounts, markups, and sales tax
• Multiply and divide with numbers in the millions, billions, and trillions
• Convert from centigrade to Fahrenheit
• Round off the right restaurant tip.

Master probability, odds, and statistics:

• What are the odds that your next two children will both be boys?
• Which games should you play when you go to a casino?
• What is the significance of polls and their margin of error and the bell curve and standard deviation?
• Do men or women have more sex partners? The answer may surprise you!

Everyday math for everyday life

• Tipping
• Balancing your checkbook
• Credit card interest
• Investments
• Recipe conversions
• Taxes and inflation
• Buying carpeting
• Baseball stats
• Gambling odds

A painless and easy guide to making sense (and cents) out of numbers! Let's face it, math is everywhere. In this practical guide, Mark Ryan takes the confusion, frustration, and-yes-fear out of math. You'll enjoy his jargon-free approach and you'll learn how you, too, can master math. Discover:

• How simple math equals big money when it comes to credit cards, investments, and insurance
• Basic geometry for life's essentials: mulch for the garden, fencing for the yard, carpeting for the living room, water for your pool
• How to take the guesswork out of cooking: from adjusting servings to converting measurements
• How to help your kids with math
• How to compute compound interest and mortgage payments
• Sports math: calculate a pitcher's earned run average or the win/loss ratio of your child's soccer team
• How to use the functions on a calculator or do simple math in your head ... and much more!

Now, even the most die-hard math-haters, mathaphobes, and the numerically impaired can do the math!

Like it or not, math is here to stay. We live in a world of percentages, statistics, interest rates, and taxes. The Dow goes up 3 percent, the NASDAQ drops 5 percent. The inflation rate is such-and-such a percent, the trade deficit is so many billions, and the national debt is so many trillions. And how big of a tip should you leave? You just can't get away from numbers.

My aim in writing this book is to give you an easy-to-follow guide to the math you encounter in your everyday life. I hope this single volume will become the first resource you turn to for all your mathematical or numerical questions. The topics range from the simple - how to solve a percentage problem, how to figure a tip, how to balance your checkbook - to the complex - the formula for the present value of an annuity, an installment loan formula, and believe it or not, a simple explanation of Einstein's famous formula, E = mc2. You will also find here important numerical facts and definitions: everything from the definitions of gross domestic product and the federal deficit to windchill and heat index tables and the meaning of barometric pressure. Regardless of the complexity of the topic, all explanations are in plain English with a minimum of technical jargon.

The first part of the book is a review of all the math topics you need in your day-to-day life. This is not a review of high school mathematics. High school math teachers often try to persuade students that what they're studying will prove useful, but let's face it, students are generally on the mark when they ask rhetorically, "When are we ever going to use this?" Few adults will ever need to use geometry proofs, trigonometry, advanced algebra, pre-calculus, or calculus. In fact, although simple algebra can come in handy, you can really get by without it.

Here are topics you do need. Basic arithmetic is useful for doing things like balancing your checkbook and dealing with big numbers in the millions and billions. Fractions, decimals, and percents have endless applications: cooking, tipping, discounts, markups, mortgage rates, investments, etc. You use ratios and proportions whenever you change the scale of something, like with a scale drawing or a map, or when you adjust a recipe for different numbers of people. You will want to have a basic understanding of powers and roots to be able to work with financial formulas, among other things.

Another fundamental is basic geometry - shapes are all around us, and every adult should understand the few simple formulas for perimeter, area, and volume, and the various units of measure for these quantities: inches, feet, miles, square feet, square yards, acres, cubic feet, gallons, etc. Sample problems include estimating how many square yards of carpeting you'll need to buy, and figuring the amount of fertilizer you need for your lawn. You need to have a basic understanding of probability and odds for gambling, dice games, card games, and the lottery, and to grasp the meaning of statements like "there's a 25 percent chance of rain tomorrow," or the often-quoted probability that a woman has a one-in-nine chance of contracting breast cancer sometime in her life. The media inundates us with medical studies, opinion polls, political polls, etc., and thus we need a basic understanding of statistics to separate the wheat from the chaff.

Do the Math

One of my objectives is to give you the tools you need to become mathematically self-reliant, so that as you go about your day-to-day affairs, you'll consider the math when making your decisions. You should know, for example, that flying from Chicago to Miami is much safer than driving. You may decide to drive anyway, for any number of reasons - the scenery, you enjoy driving, etc. - but you should know the relative accident statistics. You should be aware that if you carry a large credit card debt (at, say, a 19.8 percent APR) month after month without paying down the debt, you are throwing money down the drain at a truly alarming rate. When you do the math, you'll see that it rarely makes sense to take advantage of the "convenience" of a credit card unless you know that you'll be able to pay the balance down to zero in the near future. You may decide to buy that new computer or big-screen TV anyway, but you should be aware of the financial implications. Before you decide to buy a low-deductible health insurance plan - so that you'll "never have to worry about medical bills again" - you should know that you'll probably come out ahead if you buy a high-deductible plan. Before you go to the casino, you should know that roulette and craps are games of luck, not skill, and that the odds are against you. When it comes to math, ignorance is not bliss.

Then Ignore the Math

You should do the math, but the math won't always give you the answer you seek. Say you're buying a house and you've narrowed your choices down to two homes. The most important factors to you are location, price, the neighborhood, and the quality of the schools. You rate each of these two homes on each of the four factors, on a scale from 1 to 10, and house A comes out ahead of house B on three of the four factors, or even on all four factors, yet for some unexplained reason you still prefer house B. What should you do? Go with the math or your gut? Go with your gut. Why ignore your careful analysis? I suspect that what happens in situations like this is that we're subconsciously considering more factors or assigning them different weights than we are aware of or are able to articulate. Let's say house B came out on top only on the schools issue and only by a little. If you nevertheless prefer house B, it may be because you are aware subconsciously that the schools factor is more important than the other three combined. What if you prefer house B even when house A comes out ahead on all four factors? The likely explanation is that one or more factors that you weren't aware of or had decided weren't important or "shouldn't" be important - like a beautiful porch or the number of trees on the property - really are important to you.

Now, does this mean that your analysis was a waste of time? Not at all. If you had not done any analysis and had made your decision only on instinct, you might end up in the wrong house. Let's say you don't have any children yet but are planning to raise a family, and thus you absentmindedly neglect to consider the schools issue. If you buy the house you like and then later learn that the schools are the worst in the country, you will wish you had done a thorough analysis before buying. The moral of the story is that in complex situations like this, you should do the math and listen to your gut instincts. Both are important. You can't make an informed decision unless you do the math. And you should also listen to your instincts because, in the words of mathematician and philosopher Blaise Pascal, "The heart has its reasons."

Copyright © 2002 by The Math Center, Inc.

About the Author

A graduate of Brown University and the University of Wisconsin Law School and a member of the National Council of Teachers of Mathematics, Mark Ryan has been teaching math for over 12 years. He runs the Math Center in Winnetka, Illinois (www.themathcenter.com) where he teaches high school math courses including a course and a related workshop for parents based on a program he developed, The 10 Habits of Highly Successful Math Students. In high school, he twice scored a perfect 800 on the math portion of the SAT, and he not only knows mathematics, he has a gift for explaining it in plain English.