19/20 is 85%.
The easiest fractions can be simply solved by multiplying the numerator and the denominator such that the denominator becomes 100, then the numerator is your percent.
In this case, you simply multiply top and bottom by 5.
In the cases where the denominator does not easily go to 100, you could plug the fraction into the calculator and then multiply by 100.
Some common fractions->%s