The first thing you should do is write the matrix and start counting.In this case I wrote the matrix in excel. (see attached image).
 Of the population of 48 people, 17 people were under 20 years old.
 So, the proportion is the following.
 17/48 = 0.354166667
 The percentage is
 (17/48) * 100 = 35.41666667%