You have 7 blue tickets and 21 total.
So the odds of pulling a blue ticket is 7
21
If you replace the ticket, the total number of tickets changes, so the total is 20 tickets.
The odds of pulling a purple ticket is 10
20
So the odds that you pull a blue, replace it, then pull a purple is:
7 * 10 = 70 = 1
21 20 420 6
so your answer is 1/6 or D