If a weighted coin is tossed N times, count the times it lands heads as H. 
The probability of landing heads is P(H).

I assume the expected value of H is N*P(H).

But how do I find out what range of H is X% likely to happen?  For example, 
I want to be able to say that in 90% of cases H will be in the range (A,B).

What I've done so far is for 0<=H<=N calculated (N choose H)/P, where P is 
the total permutations of N items in N.  This should give me the probability 
of exactly H heads coming up, yes?  I can keep a cumulative total of these 
probabilites for each H, and then figure out the range of H between (say) 
10% and 90%.  But it seems a bit long-winded, I can't figure out how to 
calculate the range directly.  Also Excel can't figure out the permutations 
when N is bigger than 150 or so, I want it to work for higher values of N.

Any ideas?  Hopefully it's something obvious!

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