How to calculate percentages
A percentage is a fraction whose denominator (bottom number) is always 100. It is denoted using the percent sign (%) or the abbreviations pct. So 1% is 1/100. If we say 20%, we mean 20/100 = 1/5. So 25% means 1/4 and 50% means 1/2
If you have to turn a percentage into a decimal, just divide by 100. For example, 25% = 25/100 = 0.25
If you have to turn a percentage into a fraction, just add 100 as a denominator. For example, 50% = 50/100 = 1/2
The following examples show how to calculate percentages:
3 is what percent of 12?
$$3/12=1/4={1×25}/{4×25}=25/100=25%$$
What is 20% of 80?
$$80×{20/100}=80×{1/5}=16$$
8 is 40% percent of what number?
$${8×100}/40=800/40=80/4=20$$
The price of a book was increased from 10 to 12 USD. Express this increase as a percentage change of the original price.
$$Change = {Difference}/{Old;Value} ×100=$$
$$={12−10}/10 ×100=$$
$$=2/10 ×100 = 20%$$
The price of a book was decreased from 15 to 10 USD. By what percentage did it decrease?
$${15−10}/15 × 100=5/15 ×100=$$
$$ = 33.3%$$
