Percents

Percents are a way of standardizing measurements so they can be compared. For example, if school A has 800 students with 400 going on to college and school B has 500 students with 300 going on to college, how can you compare the two schools? Percents standardize the measurements to the number out of a hundred. (Percent means out of 100.) 50 out of every 100 in school A go to college (50 %) while 60 out of every 100 in school B go to college (60 %).
Or say you invested $500 in stock A and earned $120, and invested $2000 in stock B and earned $400. You earned more on stock B, but youu had to invest more to get it.
Converting them to %, you earned $24 for every $100 invested in stock A (24 % return), and only $20 for every $100 invested in stock B (20 % return), so A is the better investment.
While there are shortcuts for doing percent problems, they are all based upon the proportion: PART / WHOLE = PERCENT / 100.