In first draw, there are 10 black balls and 15 total balls.So,
Probablity of drawn ball is black `= 10/15 = 2/3`
In second draw, there are 9 black balls and 14 total balls.So,
Probablity of drawn ball is black `= 9/14 `
So, probablity of drawing both black balls `=2/3**9/14 = 3/7`