Hello.

Yup

No; 20C19 = 20, 0.25^(19) is nearly 0

Minus Part A? what? I thought your solution for A was 5.46%?

Anyway; you need to find

1 - prob("all 20 answers are incorrect") = 1 - prob("0 answers are correct")

where prob("20 anwers are incorrect") = 20C0 *0.25^0*0.75^20 = 0.003 = 0.3%