The word 'percent' can mean 'for every 100' or 'per 100'. And we can imagine 100% as being a whole of something.
In the question, we have 20 homework sections. 20 can be thought as the 100%. We word 'per' in 'percent' can be sought of as a division operator or the fraction line that separates the numerator and denominator.
So, we can think 18 out of 20 as

First simplify the fraction.

Convert the fraction into a decimal.
We can convert a fraction into a decimal by dividing the numerator by the denominator.
9 / 10 = 0.9
Now convert the decimal into a percent by moving the decimal point two places to the right and making the decimal point into a % sign.
0.9 = 90%
So, 90% is the answer.