What is the probability of flipping a coin 75 times and getting tails 30 times or fewer?

Question

perl · Answer

you can use binomial distribution

perl · Answer

Let X = # tails in 75 coin flips

You want to find

P( x <= 30)

perl · Answer

Let X = # tails in 75 coin flips

You want to find P( X <= 30) 

you can solve it two ways i think, using a calculator or a normal distribution approximation

perl · Answer

there are online cumulative binomial calculators

hockeychick23 · Answer

ok thanks

perl · Answer

the formula is exactly
SUM_{i=0}^30 (75 choose i)*1/2^75

hockeychick23 · Answer

oh ok i had looked up a binomial calculator online and entered the information in there, ill try it that way

hockeychick23 · Answer

ok i got 32.2%

perl · Answer

thats correct

hockeychick23 · Answer

ok thanks!

perl · Answer

wait

hockeychick23 · Answer

hmm it said it was wrong

perl · Answer

you have to copy paste that entire link

perl · Answer

here again http://www.wolframalpha.com/input/?i=SUM_i%3D0^30+%2875+choose+i%29*1%2F2^75

perl · Answer

then multiply by 100 to get percent

perl · Answer

its a long decimal, so just use the first 3 or so

hockeychick23 · Answer

ok ill use that calculator in the future, thanks