Problem Solving

Hello,

I am not able to solve the problem (attached).

Could you please help me with this?

Thanks.

Ok, so we have 15 slate rocks out of 45 rocks total.

The first time we pick a rock, we have a 15/45 = 1/3 chance of getting a slate rock.

Since we are not replacing that rock, we now have 14 slate rocks and 44 rocks total, for a 14/44 = 7/22 chance of getting a slate rock.

In order to get two slate rocks, we need to be in the lucky 1/3 that get a slate rock the first time, and then in the lucky 7/22 that get a slate rock the second time. To calculate the probability of being lucky both times, we need to multiply these outcomes together.

1/3 * 7/22 = 7/66

Hope that helps,

Tatiana

Thank You so much.

Thank you for your reply. However could you please clarify why did you multiply 1/3 and 7/22 and not add the probabilities?
Thanks