Concept: The question tests on Word Problem on Rates(Time and Work)
Complete Job by Machine X take 4 hours. Therefore half the job takes 2 hours.
Work done by X and Y together in 1 hour = 1/4 + 1/5 = 9/20.
Therefore time taken to complete it together = 20/9 hours
Time taken to do half the job together = 10/9 hours
Total time taken = 2 + 10/9 = 28/9 hours