Question
Solve the following problem :

Find the sequence that minimizes the total elapsed time to complete the following jobs. Each job is processed in order AB.

Machines Jobs (Processing times in minutes)
I II III IV V VI VII
Machine A 12 6 5 11 5 7 6
Machine B 7 8 9 4 7 8 3

Determine the sequence for the jobs so as to minimize the processing time. Find the total elapsed time and the idle times for both the machines.

Answer

Machines Jobs (Processing times in minutes)
I II III IV V VI VII
Machine A 12 6 5 11 5 7 6
Machine B 7 8 9 4 7 8 3

Observe that Min (A, B) = 3, corresponds to job VII on machine B.

∴ Job VII is placed last in sequence.

VII

Then the problem reduces to

Machines Jobs (Processing times in minutes)
I II III IV V VI
Machine A 12 6 5 11 5 7
Machine B 7 8 9 4 7 8

Now, Min (A, B) = 4, corresponds to job IV on machine B

∴ Job IV is placed before VII in sequence.

IV VII

Then the problem reduces to

Machines Jobs (Processing times in minutes)
I II III V VI
Machine A 12 6 5 5 7
Machine B 7 8 9 7 8

Now, Min (A, B) = 5, corresponds to job III and V on machine A.

∴ Job III and V is placed either first or second in sequence.

III V IV VII
OR
V III IV VII

Then the problem reduces to

Machines Jobs (Processing times in minutes)
I II VI
Machine A 12 6 7
Machine B 7 8 8

Now, Min (A, B) = 6, corresponds to job II on machine A

∴ Job II is placed on third place in sequence.

III V II IV VII
OR
V III II IV VII

Then the problem reduces to

Machines Jobs (Processing times in minutes)
I VI
Machine A 12 7
Machine B 7 8

Now, Min (A, B) = 7, corresponds to job I on machine B and VI on machine A.

∴ Job I is placed before IV and job VI on remaining in sequence.

III V II VI I IV VII
OR
V III II VI I IV VII

We take the optimal sequence as,

III V II VI I IV VII

Total elapsed time

Job Machine A Machine B
In Out In Out
III (5, 9) 0 5 5 14
V (5, 7) 5 10 14 21
II (6, 8) 10 16 21 29
VI (7, 8) 16 23 29 37
I (12, 7) 23 35 37 44
IV (11, 4) 35 46 46 50
VII (6, 3) 46 52 52 55

∴ Total elapsed time = 55 mins

Idle time for machine A = 55 – 52 = 3 mins

Idle time for machine B = 5 + 2 + 2 = 9 mins.

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