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
Tatiana Becker | GMAT Instructor | Veritas Prep
