Answer:
A.) The probability that the missile is not detected by any of n sets = (1-0.9)^n = 0.1^n.
Where it's opposite will be that the missile is detected by atleast 1 of the sets. Which will be equals to 1-(0.1)^n.
B.) 0.999= 1-(0.1)^n
Here n=3
For, 0.999999=1-(0.1)^n
n=6.
C.) If n=5
5C4*(0.9)^4*(0.1)^1= 0.32805