A percentage is a portion out of 100, and because it is a portion of a whole it can be written as a fraction of an amount (in this case a fraction of 100). You can also rewrite this as a decimal because a fraction is essentially the same thing as a division problem and when you divide a number by 100 you usually get a decimal (if the numerator is smaller than 100 that is), and also because decimals are similar to fractions because they represent parts a whole unit for instance the percentage 72% is equivalent to the fraction 72/100 which is the same as 72 divided by 100 which is the same as the decimal 0.72.
Sorry for the long explanation but I hope this helped!