Each machine completes 1/10 of the total work in 1 hour
3 work the whole 2 hours....so....they complete 2(1/10)*3 = 6/10 of the work during this time
The other two work one whole hour (between 8 and 9) without stopping so they complete another 1 (1/10) * 2 = 2/10 of the work during this time
So......we have accounted for 8/10 of the work
Between 7 and 8 the other two machines complete (53/60) * 2 (1/10) = 53/300 more of the work
So....the five machines complete 8/10 + 53/300 = 293/300 of the work in two hours
So....the five must complete 7/300 of the remaining work after 9 AM
Note that each minute.....one machine completes (1/10) (1/60) = 1/600 of the work
So..... each minute the 5 machines complete 5/600 = 1/120 of the work
So....the number of additional minutes they must work to complete the job is
Work remaining / portion of work done each minute =
(7/300) /( 1/120) = (7/300) (120) = 14/5 = 2.8 minutes = 2 min 48 sec
So....the time for completion is 9 AM + 2min 48 sec = 9:02:48