probability?

(a)
P( red marble )
= P( Box A ∩ red marble ) + P( Box B ∩ red marble ) + P( Box C ∩ red marble )
= (1/3)(6/14) + (1/3)(7/14) + (1/3)(12/21)
= 0.5 ..... Ans

(b)
P( Box C｜blue marble )
= P( Box C ∩ blue marble ) / P( blue marble )
= (1/3)(9/21) / ( 1 - 0.5 )
= 2 / 7 ..... Ans