Question 34415
We can do it with a shortcut call "combination", but I will use the long way, call "list them all out!"

Different pizzas that Paul can order:
1) Peperoni, onion
2) Peperoni, sausage
3) Peperoni, mushrooms
4) Peperoni, anchovies
5) onion, sausage
6) onion, mushrooms
7) onion, anchovies
8) sausage, mushrooms
9) sausage, anchovies
10) mushrooms, anchovies

So now we can see that 4 out of 10 pizzas have mushrooms on them, so the probability will be 40%