The probability of getting a single tails =
p = 1/2
The probability of not getting a single one =
q = 1 -p = 1/2
Now, using binomial theorem of probability:
The probability of getting exactly r = 4 tails in total n = 10 tosses =
(10 x 9 x 8 x 7)/4! * (1/2^4) * (1/2^6) = 105/2^9 = 20.51%
