Also you haven't read the question clearly as you shouldn't be trying to count selections of 2 defectives.
The lot will be accepted if 20 good units are chosen from the 460 good ones (since 40 are defective)
if 19 good ones are selected from the 460 good and 1 defective is chosen from the 40 bad.
is the probability of choosing 20 good ones.
is the probability of choosing 19 good and 1 bad together