The first decision, which is related to that measurement, is whether your goal is a distance in metric units (usually kilometers) or in English units (usually miles). If you decide that your answer should be in kilometers, it will be easier if you make the initial map measurement in centimeters. If you decide that your answer should be in miles, it will be easier if you make the initial map measurement in inches.
Once the measurement is made, the scale must be applied. Remember that the value after the colon in the representative fraction is telling you how many times bigger the real world is than the map. If that map scale is 1:24,000, then multiply the measured distance by 24,000; a measurement of (for example) 2.8 inches on the map is, thus, equivalent to an actual distance of 67,200 inches.
Of course, 67,200 inches is virtually incomprehensible to most people. That is a common reaction to that first calculation in using a representative fraction scale to determine a distance. The conversion of that value to units that are more comfortable is the next, key, step. To accomplish this next step, you call into use your own knowledge of measurement conversions, as discussed earlier. Assume that we need to stick with English units, so that you will have to convert inches to miles. Remember, there are 63,360 inches in one mile.
There are two approaches to resolving this calculation that are commonly taught. One approach says that you divide 67,200 inches by 63,360 inches per mile. The answer will be the measurement in miles. The second approach says that you multiply 67,200 inches by 1 mile per 63,360 inches. The answer is the same; the only difference between the two is that one or the other will mesh with how you have been taught and what you feel comfortable with. The key to success using either approach is keeping track of your units of measure as you proceed through the calculations.