Ratios, percentages, proportions and rates are all fractions. Therefore, jobs that use fractions are ubiquitous. Because currency is divided into fractions, any job that uses money uses fractions. Anyone who calculates tax, like a cashier, is using fractions. Less trivial examples include any engineering job, many health-related and business jobs, and all science jobs.
Rates
Any job that uses rates uses fractions. For example, shipping rates are determined by weight or volume of the package being measured. The rate is a fraction in units of dollars per pound or dollars per cubic inch. A long-distance trucker who needs to complete a trip distance within a certain time might need to figure out his required minimum speed using rates.
Percentages
Any job that uses percentages uses fractions, since a percentage is a ratio formed with the number 100. Therefore, any business involving tax calculation, tip calculation, or interest rates uses fractions. Banks, restaurants, movie theaters and department stores all use percentages, so teller, wait staff and store clerk positions are included here.
Health
Medical equipment measures ratios and rates (for example, blood pressure and pulse). Prescription dosages are based on a ratio of medicine to body mass and to frequency of ingestion, itself a rate. Body-mass index is a ratio of height to weight used by doctors to judge fitness. Pharmacists, medical doctors and health staff must therefore be familiar with ratios and rates.
Engineering
Engineering studies how variables in physical systems vary in proportion to each other. Therefore, engineers are steeped in fractions (proportions). Every engineering field uses fractions, from stress-to-strain ratios to chemical concentration ratios and reaction rates to ratios in electrical equations to solve for current and voltage.
Science
Fractions are used everywhere in science: from radioactive decay rates to statistical analysis to anything using calculus (the study of rates of change). Even in biology, counting proportions of cells of a certain character, counting changing proportions of a population affected by disease, and pretty much any intersection of chemistry with biology uses fractions. Nearly every job in science uses fractions of some sort.
Cooking
In cooking, the ingredients are often measured in fractions of units. Recipes are often reduced to a portion of the original recipe, which involves finding fractions of the original ingredient measurements. Chefs, cooks and dietitians all use fractions.
Farming and Car Mechanics
A farmer deals in measures of rainfall and fertilizer, and how that relates to harvest and market prices. These relations form ratios, which are used to determine purchase and harvest schedules. And because farmers must be good businessmen, farmers are exposed to the use of fractions in the business world (interest rates, tax calculation, and so forth).
A car mechanic also deals in fractions. For example, a differential pulley--a tool used by mechanics to lift engines--depends on two pulleys having similar radii. The ratio between the two determines the mechanical advantage. Maintenance work like tune-ups (such as replacement of spark plugs) aims to reduce rates of gas consumption, which are themselves fractions. And mechanic's tools are measured in fractions of inches and meters.
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