Basic Math and Pre-Algebra For Dummies (74 page)

Any temperature below 0°C is cold, and any temperature over 30°C is hot. Most of the time, the temperature falls in this middling range. So now you know that when the temperature is 6°C, you want to wear a coat. When it's 14°C, you may want a sweater — or at least long sleeves. And when it's 25°C, head for the beach!

Many books give you one formula for converting from English to metric and another for converting from metric to English. People often find this conversion method confusing because they have trouble remembering which formula to use in which direction.

In this section, I show you a simple way to convert between English and metric units that uses only one formula for each type of conversion.

 Here's a nice pair that's easy to remember: 16°C is about 61°F.

When you multiply any number by 1, that number stays the same. For example, 36 × 1 = 36. And when a fraction has the same numerator (top number) and denominator (bottom number), that fraction equals 1 (see Chapter
10
for details). So when you multiply a number by a fraction that equals 1, the number stays the same. For example:

If you multiply a measurement by a special fraction that equals 1, you can switch from one unit of measurement to another without changing the value. People call such fractions
conversion factors.

Take a look at some equations that show how metric and English units are related (all conversions between English and metric units are approximate):

Because the values on each side of the equations are equal, you can create

When you understand how units of measurement cancel (which I discuss in the next section), you can easily choose which fractions to use to switch between units of measurement.

When you're multiplying fractions, you can cancel any factor that appears in both the numerator and the denominator (see Chapter
9
for details). Just as with numbers, you can also cancel out units of measurement in fractions. For example, suppose you want to evaluate this fraction:

You already know that you can cancel out a factor of 2 in both the numerator and the denominator. But you can also cancel out the unit
gallons
in both the numerator and the denominator: