Question 34415
This is an A or B Probability Question. We want to know the probability of picking mushroom as a first topping or a second topping. 

Probability of picking mushroom as the first topping is 2/5 (We'll call this A)
Probability of picking mushroom as the second topping is 1/4 (We'll call this B)

The Probability of A or B Formula is A + B - A x B

A + B = 2/5 + 1/4 = 4/20 + 5/20 = 9/20
A x B = 1/5 x 1/4 = 1/20
A + B - A x B = 9/20 - 1/20 = 8/20 = 4/10 = 2/5