Probability of many coin flips coming up heads 65% etc.

Hello, I am wondering how to calculate what the probability would be of flipping a coin, say, 250 times and having it come up heads 65% of the time. Or 500 times and 90%. Maybe I should be asking how to calculate if it will be at LEAST 65%, at least 90%, etc. I am very interested in this problem.

EDIT: Where the number of trials n is large and the per-trial probability p does not differ hugely from 0.5, you can use the normal approximation to estimate the probabilities you’re after with good accuracy.

Last edited by Con-Tester; 08-27-2015 at 02:40 AM.
Reason: More info.

Re: Probability of many coin flips coming up heads 65% etc.

http://stattrek.com/online-calculator/binomial.aspx
here is an online automatic calculator for you. Just fill in the probability of success (0.5), number of trials (250), number of successes (65% x 250 = 162.5 -> 163).
AT LEAST = P (X >=163)