Question

On an average 2 out of 100 bulbs give you power of less than 500 watt.What is the probability that out of the batch ( which is of 100 bulbs ) 2 of them give you less than 500 watt power?

Answer 1

chance for any combination with 2 bulbs satisfying 2% condition is 0.02^2*0.98^98, and it should be multiplied by number of such combinations in 100 bulbs, which should be (99+98+97+ ... 1) because it can be either first one and any of other 99, then second one and any of the 98(not taking into account first one because 1 and 2nd bulb is in list when we try options for first bulb), 97, and so on.

I am not sure why its this way(if it is at all), just logic :-)

Answer 2

I would appreciate if you would let me know if I am right! :-)

Answer 3

firstly, srry i did not respond...
secondly, the logic is that
1) lets say that giving less than 500w power was "being" defective{ so as to easily write it here }
2) now those two bulbs have to be out of the defective ones
=> {(2C1)/(100C1)}* {(1C1)/(99C1)}

thts the answer...