Basic Math and Pre-Algebra For Dummies (85 page)

Another way you can make this problem a little easier is to notice that the number 24 is in the numerator and 3 and 8 are in the denominator. Of course, 3 × 8 = 24, so you can cancel out all three of these numbers:

At this point, the expression has only two numbers left aside from the 1s, and the fraction's easy to simplify:

As always, the conversion chain from 24 furlongs to 2,640 fathoms doesn't change the value of the expression — just the units of measurement.

Sometimes real-life measurements just aren't that accurate. After all, if you measure the length of a football field with your trusty ruler, you're bound to be off an inch or two (or more). When you perform calculations with such measurements, finding the answer to a bunch of decimal places doesn't make sense because the answer's already approximate. Instead, you want to round off your answer to the numbers that are probably correct. Here's a problem that asks you to do just that:

Heather weighed her new pet hamster, Binky, and found that he weighs 4 ounces. How many grams does Binky weigh, to the nearest whole gram?

This problem requires you to convert from English to metric units, so you need this conversion equation:

Notice that this conversion equation includes only kilograms and pounds, but the problem includes ounces and grams. So to convert from ounces to pounds and from kilograms to grams, here are some equations to help build a bridge between ounces and grams:

Your chain will perform the following conversions:

So set up your expression as follows:

As always, after you set up the expression, you can cancel out every unit except for the one you're converting to:

 
When you're multiplying a string of fractions, you can make one fraction out of all the numbers. The numbers that were originally in the numerators of fractions remain in the numerator. Similarly, the numbers that were in the denominators remain in the denominator. Then just put a multiplication sign between each pair of numbers.

At this point, you can begin calculating. But to save some effort, I recommend canceling out common factors. In this case, you cancel out a 4 in the numerator and denominator, changing the 16 in the denominator to a 4:

Now you can cancel out another 4 in the numerator and denominator, changing the 1,000 in the numerator to 250:

At this point, here's what's left: