This is a probability problem and I need help. I'm in Mathematical Foundation for computer science and it's like discrete math

[10] 1. A certain company owns two machines that send numbers 1, 2, or 3 each time they they are activated. Machine A sends each of the three numbers with equal probability. Machine B sends 1's with with 50% probability and 2's and 3's with 25% probability cach. A machine is chosen at random and activated seven times sending 1,3,3,2,1,2,1. What is the probability that the machine chosen was machine B