Divide the numerator by the denominator if you're given a fraction. You may also have to deduce the fraction from a word problem. For example, if you're told that Billy got 80 out of 90 questions right on his math exam, you can determine a percentage by first dividing the questions he got right (80) by the total number of questions (90). The result would be 0.89.
Multiply the quotient you calculated by 100 to get a percentage. In the example, you would multiply 0.89 by 100 to get 89 percent. You could also write the answer as "89%."
Subtract the smaller number from the bigger number and divide that answer by the original one to get a percentage increase or decrease. For example, if your interest-bearing savings account at the beginning of the year had $1,000 and it stood at $1,050 by the end of the year, you can subtract $1,000 from $1,050 to get $50. Divide that by the original amount, $1,000, to get 0.05. Multiply the quotient by 100 to get a percentage increase of 5 percent.