Slide 48 of 61

Solution:

Let x represent the number of Professor McGonagall cards

x + 5 represents the number of Professor Dumbledor cards

(1/5) x 100 = 20, the number of Professor Quirrel cards

(1/6)x represent the number of Harry Potter cards

2 x (1/6)x represents the number of Hagrid cards

100 = x + (x + 5) + 20 + (1/6)x + (2/6)x

x = 30

There are 30 Professor McGonagall cards.