Box contains 4 white balls and 8 black balls. Pick a ball with your left hand, and don't look, pick a ball with your right hand, and don't look at it either.

a. what is the probability the ball in your left hand is white?

b. Next, you look at the ball in your right hand and it is black. Now what is the probability that the ball in your left hand is white?

a. 4/(4+8)= 4/12 = 1/3 => 33,3%

b. 4/(4+7)= 4/11 => 36,4%