Hi! I need help with the two questions below. Thanks!

a) Xiao Ming and four of his friends were hired for a job. If Xiao Ming got one more friend to join the team, the job can be completed 6 days earlier. Assuming all of them work at the same rate, how many friends should Xiao Ming ask to join the original team so that they can complete the job 12 days earlier?

b) It is given that x is directly proportional to y and y is directly proportional to z. Prove that x + y is directly proportional to z