For every 1000 light bulbs, 5 are defective. What is the probability of buying a working bulb?

The probability of any event is equal to the ratio of the number of outcomes favorable to this event to the total number of all equally possible incompatible elementary outcomes forming a complete group.
In this problem, the event of interest to us is the choice of an unreflected light bulb.
Unframed bulbs 1000-5 = 995 – this is the number of favorable outcomes.
1000 is the number of all equally possible incompatible elementary outcomes.
P = 995/1000 = 0.995
Answer: 0.995

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