80% of all scheduled trains through New York station arrive on time. If 10 trains go through the station every day, find: the probability that at least 8 trains are on time for 9 out of the next 10 days

Question

anonymous · Answer

@satellite73

anonymous · Answer

n=10 trains , p=4/5 but the last bit i dont get. Could you please help

anonymous · Answer

lol maybe you have to use the binomial twice !!

anonymous · Answer

at least 8 trains are on time for one day is 
$$\binom{10}{8}(.8)^8(.2)^2+\binom{10}{9}.8^9(.)+.8^{10}$$

anonymous · Answer

i get $.6778$ rounded http://www.wolframalpha.com/input/?i=%2810+choose+8%29.8^8*.2^2%2B10%28.8%29^9*.2%2B.8^%2810%29

anonymous · Answer

then finish with 
$$\binom{10}{9}(.6778)^9\times .3222$$