Initial number of students: 220
Final number of students : 180
Multiply the initial number of students (220) by the percentage change in decimal form (divided by 100) - That expression must be equal to 180.
220 x = 180
Solve for x:
x=180/220
x= 0.818181
multiply by 100
81,81 %
100%-81,81% = 18.18%
Percent change: 18.18%