Since, the set of three consecutive numbers from 1 to 20 are (1, 2, 3), (2, 3, 4), (3, 4, 5), ..., (18,19,20)
Considering 3 numbers as a single digit
∴ the numbers will be 18
Now, we have to choose 3 numbers out of 20. This can be done in 20C3 ways
∴ n(S) = 20C3
The desired event is that the 3 numbers are choose must consecutive. So,
Hence, the correct option is (B).