P(B)is using "marble pick" as event and color order matters; whereas P(A&B) is using "bag pick" as event marble color picking order does NOT matter.
P(A&B) using "marble pick" as event and color order matters={白白}/ {白白,白黑,黑白,黑黑}=1/4
P(A|B) = P(A&B)/P(B)=(1/4) /( 1/2)=1/2 !