How do you convert fractions to decimals and percentages?

The single operation is division: the fraction bar means divide, so the numerator divided by the denominator gives the decimal. Three quarters is 3 divided by 4, or 0.75. To get a percentage, multiply the decimal by 100, or equivalently move the decimal point two places right, giving 75 percent. Going backward, a percentage becomes a decimal by moving the point two places left, and becomes a fraction by putting it over 100 and reducing, so 40 percent is 40/100, which reduces to 2/5.

Memorizing a small set covers nearly all practical use: 1/2 is 0.5 or 50 percent, 1/3 is about 0.333 or 33.3 percent, 1/4 is 0.25 or 25 percent, 1/5 is 0.2 or 20 percent, 1/8 is 0.125 or 12.5 percent, and 1/10 is 0.1 or 10 percent. Note that thirds and sevenths produce repeating decimals, since a fraction terminates only when the denominator's prime factors are just 2s and 5s, which is a genuinely useful thing to know when a calculator gives you an endless string. In the kitchen and workshop, where fractions dominate, the reverse conversion matters most: 0.375 on a caliper is 3/8, and recognizing eighths and sixteenths by their decimals saves considerable time.