We can rephrase the question as "What percent of 50 is 20?".
"What percent ..." is an unknown. We'll use the variable x to represent this unknown.
The word 'of' in mathematics can mean 'multiply'. Likewise the word 'is' can mean 'equal'.
Now we have the phrase "20 equals to x times 50".
Or in an equation: 20 = x * 50
Solve for x.
20 = x * 50

= x <-- Divide both sides by 50
Simplify the fraction.

Convert the fraction into a decimal.

= 0.4
Convert the decimal into a percent.
0.4 = 40%
So, 40% is the answer.