probability to select any one box among three:P(B1)=P(B2)=P(B3) =1/3
prob. of drawing one white and one red from box B1 =C(1,1) * C(3,1)/C(6,2) (Let's say P(A/B1))
prob of drawing one white and one red fromm Box B2 =C(2,1)*C(3,1)/C(9,2) (Let's say p(A/B2))
prob. of drawing one white and one red from Box B3 = C(3,1) * C(4,1)/C(12,2)(Let's say P(A/B3))
Total Probability =P(B1)* P(A/B1) +P(B2)* P(A/B2) +P(B3)*P(A/B3)
From Baye's theoram
prob. that these two balls are drawn from box B2
P(B2/A)
=P(B2)*P(A/B2) / (TOTAL PROBABILITY) u can put and check:):)