Originally Posted by

**hansel13** I never took Stats before, so I'm just trying to get a grasp of statistics and make sure I understand all of it.

A box in a certain supply room contains four 40-W lightbulbs, five 60-W bulbs, and six 75-W bulbs. Suppose that three bulbs are randomly selected.

(c) What is the probability that one bulb of each type is selected?

$\displaystyle

P = {{\binom{4}{1}} {\binom{5}{1}} {\binom{6}{1}}}/{\binom{15}{3}} = .264

$

(d) Suppose now that bulbs are to be selected one by one until a 75-W bulb is found. What is the probability that it is necessary to examine at least six bulbs?

$\displaystyle

P = {{\binom{6}{0}}{\binom{9}{6}}}/{\binom{15}{6}} = .017

$

Hope I did these right! Part d is especially tricky so I wouldn't be surprised if I tripped over on that one.