How do you write 18/20 a percentage

18/20 is equal to 90%.

When you say "Percentage", it means automatically that it's a number/100 
here you have 18/20, to make the percentage, you need to make it /100 
100/20 = 5
so you need to multiply the the numerator and denominator of 18/20 by 5, which gives you 90/100, which means 90%
Hope this Helps :)