Question

A bin contains 6 purple pens, 5 red pens, and 4 green pens. You randomly select a pen. What is the
probability of selecting a red pen?


the answer to that is 1/3

Using the fully-stocked bin from the above question, what is the probability of selecting a green
pen on Monday, then selecting a purple one on Wednesday, to replace the green one you lost
Tuesday?



a) 3/7
b) 4/35
c) 8/75
d) 3/4

Answer 1

I just need a refresher on how to do this :)

Answer 2

On Monday: fully stocked:
6 purple, 5 red, 4 green = total of 15 pens.
Prob that you choose a green pen: 4 out of 15 = 4/15

On Wednesday: not fully stocked..
6 purple, 5 red, 3 green = total of 14 pens
Prob that you choose a purple pen: 6 out of 14 = 6/14 = 3/7

And probabilities multiply :
probability of green pen then purple (without replacement of original green pen) =
(4/15) * (3/7) = (4 * 3) / (15 * 7) = 4/35

Answer 3

Thanks @cruffo

Answer 4

Cheers!