Math Riddles

Question: There are 6 sisters. Each sister has 1 brother. How many brothers are in the sisters' family?

Question: In a house of Kardak's family each brother have a sisters as double as their brothers and each sister have brothers as same as their sister. How many brothers and sisters in Kardak's family ?

Question: what does 1+1=.

Question: A man wants to have a party in thirty-one days where he will be serving his 1000 barrels of wine. The only problem is that one of his enemies poisoned one of the barrels. The poison kills any man who drinks any of the wine in about 30 days, give or take a few hours. The man has 10 plants that are also killed by the poison in 30 days and can be used to test the wine. How can identify the single poisoned barrel of wine?

Question: What are the next two letters in the following series and why? WATNTLITFS__