You're dealing with binomial distribution where n=4, and p=0.05.
So you have to find with
At least one defective case can be found using probability of the complement
Question
5% of all batteries produced are defective. Batteries are sold in packs of 4.
1)What is the probability that a pack contains exactly 2 defective batteries?
2)Atleast 1 is defective?
My working
P (def) = 0.05 P (not def) = 0.95
P (2def) = 0.05 x 0.05 = 0.0025 *wrong
Alternative route > (5/100) x (4/99) = 0.00202 *wrong