.8
All percents are out of 100, so to figure out the answer you must make the denominator, or bottom number in your fraction, 100.
To get to 100, you would multiply 5x20, because that equals 100. Because you multiplied the denominator, which is 5, you must multiply the top number by 20 as well.
4x20 = 80
So therefore, the answer is 80/100, so .8