A student moving into a new flat buys a box of 20 light bulbs. Unknown to the student, exactly one of the bulbs is defective. Find:
b) The conditional probability that the second bulb used is not defective, given that the first bulb used is not defective;
If the first bulb is not defective then you have 19 bulbs from which you can choose, and one of these is defective so the probability is .
This is the easy way to work it out but I am having trouble using the actual formula which may come in handy for other examples:
Let A be the event that the 2nd bulb is not defective.
Let B be the event that the 1st bulb is not defective.
but how can you work out ?
So how can we work out that from what we are given?
I apologise for any offence my pedantry has caused