How Many Tails?

you cannot ask a question "how many tails will come up"

that should be probability question, like "A coin is tossed 20 times. What is the probability there are 7 tails?"

The probability of getting k tails (or heads as probability for either of them is equal i.e.
1/2) out of n trials (tosses) is given by
P(n,k)=nCk(1/2)^k(1/2)^(n−k) where nCk=n!/(k!(n-k)!)
n=20, k=7

P(20,7)=20!/(7!(20-7)!)(1/2)^7(1/2)^(20−7)

P(20,7)=20!/(7!(13)!)(1/2)^7(1/2)^(13)
P(20,7)=4845/65536=0.0739288330078125

 

This is a problem I found at mathisfun.com. This is not my creation.

 

it's better to use a book, people are posting such of incomplete questions all the time (even on algebra.com)

 

I'll remember to do so.