In a lot of 12 microwave ovens, there are 3 defective units. A person has ordered 4 of these units and since each is identically packed, the selection will be random. What is the probability that
(i) all 4 units are good               (ii) exactly 3 units are good             (iii) at-least 2 units are good

P(i)= 4/(12-3)

P(ii)= 3/(12-3)

P(iii)= 2/(12-3)

I'm not sure about the answers but I answered based on my understanding!

If they are wanting all the units good (i.e. 4 out of... ,3 out of... ,2 out of...) so we have to subtract the bad ones from all the units and the that no. should be divided by the amount of units that are needed good!