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Exercises · Q12

Q.A three-machine (M1→M2→M3M_1 \to M_2 \to M_3) sequencing problem can be converted to a two-machine problem using Johnson's rule if:

(a) min⁡Ai≤max⁡Bi\min A_i \le \max B_i
(b) min⁡Ai≥max⁡Bi\min A_i \ge \max B_i or min⁡Ci≥max⁡Bi\min C_i \ge \max B_i
(c) max⁡Ai≥min⁡Bi\max A_i \ge \min B_i
(d) all processing times are equal
Maharashtra MsbshseTextbookSubjectiveImportance★★★★★
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Rule. For nn jobs on three machines M1→M2→M3M_1 \to M_2 \to M_3 with times Ai,Bi,CiA_i, B_i, C_i, Johnson's two-machine method may be applied (via Gi=Ai+BiG_i = A_i + B_i, Hi=Bi+CiH_i = B_i + C_i) only if at least one of

min⁡iAi ≥ max⁡iBiormin⁡iCi ≥ max⁡iBi\min_i A_i \ \ge\ \max_i B_i \qquad \text{or} \qquad \min_i C_i \ \ge\ \max_i B_i

holds. This ensures the middle machine M2M_2 is never the bottleneck, so the two-machine optimum is also optimal for three machines.

Options.

  • (a) min⁡Ai≤max⁡Bi\min A_i \le \max B_i — this is (almost always) true and guarantees nothing; wrong.
  • (b) matches the rule exactly — correct. …

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