A box contains seven 40W bulbs six 60W bulbs and eight 75W b

A box contains seven 40-W bulbs, six 60-W bulbs, and eight 75-W bulbs. If bulbs are selected one by one in random order, what is the probability that at least two bulbs must be selected to obtain one that is rated 75 W? (Round your answer to three decimal places.)

Solution

9 bulbs that are NOT 75W
6 bulbs that are 75W

Now, I\'m going to assume that you keep selected bulbs one by one (without replacement) until you find a 75 W bulb, at which point you stop.
Total bulbs here are 22

out of which 8 are 75w

and 13 are not 75w

we have to take out atleast 2 bulbs, which would mean that the 1st one was NOT 75W. The probability this is 13/22 = 13/22.

after thus the 2nd one will be 75w

probability oe picking up atleast two to get one 75w = 13/22