A pharmaceutical company produces boxes of tablets for a particular disease. Each box contains 20 tablets. The quality control unit of the company selects a box at random and examines the tablets to see if any of them are defective. If a particular box contains two defective tablets, what is the probability that

(i) the two defective ones are found in the first three tablets examined?

(ii) the third tablet examined is defective?

(iii) the third tablet checked is the second one that is found defective?