Learn exactly how fractions, decimals, and percentages relate to each other and how to convert freely between them.
Depending on the context, a number might be easier to interpret as a fraction, a decimal, or a percentage. Half of a pie is clearly 1/2. Calculating change at a cash register uses decimal money, like $0.50. Pointing out battery life intuitively uses percentages, like 50%.
To change a fraction to a decimal, recognize that the fraction bar simply means "divided by". For 3/4, divide 3 by 4 using long division or a calculator to get 0.75.
The word "percent" literally translates to "for every 100". Thus, you turn a decimal into a percentage by multiplying by 100. This has the effect of moving the decimal point exactly two places to the right. So 0.75 becomes 75%.
To go in reverse, take the percentage and place it as the numerator over 100. If your percentage is 40%, write 40/100. Then simplify the fraction to its lowest terms by dividing the top and bottom by their greatest common divisor (which is 20 in this case). The answer is 2/5.