Oh well, here is the explanation then
favorable outcomes when 10 and 20 are included
: 18C3 --> Because you have only 3 more numbers (out of 5) to pick from the 18 remaining numbers since 10 and 20 are already out.
Total favorable outcomes
: 20C5 --> # of ways to pick 5 items out of 20 where order does not matter
Probability of picking 5 numbers which include #10 and #20
Guys, try to explain the answer to fellow members who request such help. Simply saying the answer or "I agree with the answer" does not add to the discussion. Thanks.