A box contains four 40-W bulbs, ﬁve 60-W
bulbs, and six 75-W bulbs. If bulbs are se-
lected one by one in random order, what is
the probability that at least two bulbs must be
selected to obtain one that is rated 75W?

Question

A  box  contains  four  40-W  bulbs,  ﬁve  60-W
bulbs,  and  six  75-W  bulbs.    If  bulbs  are  se-
lected  one  by  one  in  random  order,  what  is
the probability that at least two bulbs must be
selected to obtain one that is rated 75W?

anonymous · Answer

so first one is not a 75W bulb:
P(not a 75Wbulb)=(4+5)/(4+5+6)=9/15
P(a 75W bulb)=6/(15-1)=6/14
9/15*6/14=9/35