To find out what a percentage written as a fraction would look like, you first need to convert it into a decimal.
To convert a percentage to a decimal, you either need to divide the percentage by 100, or (my personal shortcut) just move the decimal point over two places to the left.
250% = 2.5
The decimal .5 is equivalent to 1/2, since 50 is half of 100.
250% written as a fraction (well, a mixed number, really) is 2 1/2.
Hope that helped =)