HELP! why whenever i answer a question ,my best answer percentage decreases.?

That's because the Best Answer % is a simple ratio that measures the amount of Answers which are Best Answers verses those which aren't, and it also includes answers to open questions.

Therefore whilst you have a low volume of answers then your BA % will fluctuate. The more answers you have then new answers will have less impact on your overall Best Answer %

Just do the math

%BA = 100 X # of BAs / # Total Answers

So if your # Total Answers increases with no additional BAs, the number goes down.

So if you have 1 BA and 5 answers, it is 1/5 X 100

Add 5 more non-BA answers, it is 1/10 X100

Add 990 more non-BA answers, it is 1/1000 X 100

The number goes down and down. Just look at your profile at any time and do the math.

Because of simple math. BA to total answers has decreased when you answer and dont have BA for it.

% best answers is calculated on the basis of best answers out of total number of answers given. eg. if you have answered 10 questions and you got one answer as a best answer. Then your best answer percentage is 10. Now if you answer one more question then total answeres will be 11 and best answer is one. So percentage comes down to 9.09.

The percentage will definitely increase when you continue to answer more best questions.

since you have very few itwill go up when you get a best answer and down when you answer one
until you get a few hundred it will change many times

Because you are adding to the percentage.. Until you get more best answer percentage, it'll go down as the answers percent goes up. Get it?