Black and white ball problem in probability theory

Sort out, 20 balls, from 1 to 20, of which the tenth ball is required to be a black ball.

General scheme number C20 8 (8 out of 20) =1258970.

Excluding the tenth grid and a black ball, the total number of schemes is c198 (8 in19) = 755382.

The probability of the tenth black ball is C 19 8/C20 8=0.6.