Percentages are calculated by finding what portion a fractional amount takes up out of 100. By comparing a fractional amount to x/100, a percentage can be found.
"Percent" comes from the latin word for "out of (per) 100". If there were 100 of some item, and a person removed 10 items, that person would have taken 10% of the items - they would have taken 10/100. For items which do not equal exactly 100, they can be compared to it. Take for instance a group of four people, in which three are female and one is male. 3/4 people are female, and 1/4 people are male. By multiplying 4 (the denominator) by 25, we get 100. Afterwards, 3 and 1 can be multiplied by 25 to give 75 and 25; these are the percentages of men and women in the example.