Answer:
a) 
b) 0.6957
Step-by-step explanation:
Let X represent the number of 4's when  n = 5 independent spins 
each has a probability of 0.2 (i.e p = 0.2)
This notation is represented as:
X  Binomial (n = 5, p = 0.2)
 Binomial (n = 5, p = 0.2)
Probability of   number of 4's is:
  number of 4's is:

here;  is the combinatorial expression
 is the combinatorial expression
 =
 = 


So; let's first find:







b) 
Given that:
The ratio of boys to girls at birth in  Singapore is quite high at 1.09:1
What proportion of Singapore families with exactly 6 children will have at least 3 boys?
Probability of having a boy =  = 0.5215
 = 0.5215
Binomial Problem with n = 6
P(3<= x <=6) = 1 - P(0<= x <=2)
= 1 - binomial (6,0.5215,2) 
= 0.6957