The number of students in Mrs. Patrick's piano class decreased by 15% in 2 years to 17 students. How many students did she have 2 years ago?

20 students

Step-by-step explanation:

If the class decreased by 15%, the students that she has now (17) represents a percentaje of:

100% - 15% = 85%

so the 17 students are 85% of what she had:

Students Percentage

17 ⇒ 85%

and we are looking for how many students she had 2 years ago, thus we are looking for the 100% of students (the original number of studens). If we represent this number by x:

Students Percentage

17 ⇒ 85%

x ⇒ 100%

and we solve this problem using the rule of three: multiply the cross quantities on the table (17 and 100) and then divide by the remaining amount (85):

x = 17*100/85

x = 1700/85

x=20

2 years ago she had 20 students