A coin is tossed 9 times. What is the 
probability that the coin will land heads 
at least 7 times?

This is a binomial probability. You can 
find it directly from a binomial
table, a TI-83 calculator, or entirely 
from the formula.

Using the formula: 



gives the probability that in n trials 
there will be exactly x successes
where

p = the probability of one success in 
one trial and q = 1-p = the probability 
of one failure in one trial.



In the case of a coin, p = 1/2 and q = 1 - 1/2 = 1/2

P(coin landing heads at least 7 times out of 9 tosses) =

P(coin landing exactly 7 times out of 9) +   
       P(coin landing exactly 8 times out of 9) +
              P(coin landing exactly 9 times out of 9) =

 =

 =

Factor out 

 =



which gives the desired probability.

To calculate this we need:







Therefore the desired probability =

 

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If you use a TI-83 or TI-84 calculator, type in

1 - binomcdf(9,1/2,6)

then press ENTER and you get .08984375

To get binomcdf( , press 2nd, then VARS, then 
scroll down to binomcdf( , then press ENTER.

Edwin