Question

a box contains 12 bulbs of which 5 are defective. A man selects three bulbs at random. what is the expected number of defective bulbs in his selection

Answer 1

It may be 1, 2 or 3 defective bulb for his selection. 
Because he select three bulbs so combination like:
Total   Not-Defective Defective
3           1                       2
3            2                       1                  
3             3                       0