Answer:
B) 2 hours
Step-by-step explanation:
If machine  A complete a job in 3 1/2 hours or 7/2 of an hour
means that in one hour finished 1÷ 7/2   or  2/7
If machine  B complete a job in 4 2/3 hours or  14/3 of an hour
means that in one hour finished 1÷ 14/3   or  3/14 of an hour
Then the two machines working together in one hour will make
2/7 + 3/14 Â Â = Â (4 + 3)/ 14
or  7/14  = 1/2
half of the job. Therefore these two machines working together will take two hours