Arrangements

n! / (n-k)!・k!

p(heads)=0.5
flip coin 5 times
p(#head)=1
5!/4!*1! = 5, 2^5 = 32
0.15625

probability p(heads)=0.8
frip coin 3 time p(#heads= 1)
0.096

5*(0.8)^4 *(0.2)^1