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Please remember that Maximum portability is 1.

So we can get total probability of non defective bulbs and subtract it form 1 to get total probability of defective bulbs.

So here we go,

Total cases of non defective bulbs: 16 C 2 = 120

Total cases= 20 C 2= 190

Probability= 120/ 190

Probability of at least one defective= 1- (12/19)= 7/19

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