So,
75 students decreased by 80%. That means that 100 - 80 = 20% remain.
To find 20% of 75 students, just find 10%, which means simply moving the decimal point one place to the left.
75 --> 7.5 = 10% of 75
Therefore,
15 = 20% = 75 because 7.5 * 2 = 15
There are 15 students left.