probability question-don't undertand

• 787180

  10 Apr 09, 10:06

  I have to find two light bulbs from a collection of 12,all but 3 of which are working.If I test each one in turn,what is the probability that I would find two working bulbs a) among the first three bulbs tested b) when three bulbs have been tested,but not before? part a) no problem,I can get by finding P(w,w,w")x3 ie probability ( first bulb working(w),2nd bulb working(w) and 3 rd bulb not working(w") part b) what do U mean 3 bulbs have been tested but not before?..Don't know even how to start ..plse assist..thanks a million
• Mikethm

  10 Apr 09, 11:49

  b) P(WNW)+P(NWW)

• 787180

  10 Apr 09, 14:56

  But why P(WWN) not included...can explain what's meaning of "when 3 bulbs have been tested before but not before'.Thanks again
• HyuugaNeji

  10 Apr 09, 15:02

  It means you need to test it 3 times before you get 2 working light bulbs so mike's ans is correct.