probability question-don't undertand

787180 SG

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&#8221 Wink x3 ie probability ( first bulb working(w),2nd bulb working(w) and 3 rd bulb not working(w&#8221 Wink

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 SG

b) P(WNW)+P(NWW)

787180 SG

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 SG

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