I'm going to try this without reading the other answers.
1. What relation is your niece's brother to you? Nephew
2. How many birthdays does the average person have? Hm. I'm going to say one because you're only born once.
3. How many months have 28 days? All of them
4. Is it legal in Scotland for a man to marry his widow's sister? No, because he's already dead
5. Divide 30 by 1/2 and add 10. Seventy
6. If there are 3 apples and you take away 2, how many do you have? You have two. You took two.
7. A doctor gives you three pills and tells you to take one every half hour. How long do the pills last? Tricky question. The pills last half an hour each individually, and an hour and a half total.
8. A farmer has 17 sheep and all but 9 die. How many are left? Nine
9. How many animals of each sex did Moses take on the ark? One of each
10. How many two cent stamps are there in a dozen? A dozen
11. You are participating in a race and overtake the second person. What position are you now in? Second
12. Take 1000 and add 40. Now add another 1000. Now add 30. Add another 1000. Now add 20. Now add another 1000. Now add 10. What is the total? 4100.
13. Mary's father has five daughters. 1. Nana, 2. Nene, 3. Nono, 4. Nini. What is the name of the fifth daughter? MARY. Duh.
PS Crap! Of course Moses didn't take any animals on the Ark...it was Noah! Dammit.