P(head for one coin) = 1/2

so for 6 times head P(head) = (1/2)^6 = 1/64

mean = np = 6400 x 1/64 = 100   (where n=6400 and p = 1/64)

Now, poisson distribution formula is = (e^-m) (m^x) / x!

so, poisson distribution of 6 head x times = (e^-100) (100^x) / x!