Need help on Probability

[Kemie]

If 2 light bulbs was taken from a collection 12, all but 3 are working.

Find the probability that 2 are working bulbs:

a) among first 3 bulbs tested

b) when 3 bulbs have been tested, but not before

 

 

Thanks for all help.

[Titanium]

dun understand ur qns leh... Maybe my angmo not gd leh.

[Apollo]

Just guessing, me only parmpee 5 hor

 

Working bulb = 9/12 = 75%

Failed bulb = 25%

 

a) 1st bulb = 1 - 0.25 = 0.75

2nd bulb = (11/12) - 0.25 = .667

3rd bulb = (10/12) - 0.25 = .583

 

Avg probab = (0.75+.667 + .583)/3 * 0.75

= (0.667) * 0.75

= 0.500

[Mivec69]

a) 3(9/12)(8/11)(3/10) = 27/55

 

b) 2(9/12)(3/11)(8/10) = 18/55