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Virtual machines that are charged only for the time used, at ¥40 per hour (a made-up price), are used. Normally 2 machines run throughout the month (30 days), and on 20 weekdays 4 more are added for 6 hours a day to match the load. Which of the following is the charge for the month?

  1. A¥76,800
  2. B¥86,400
  3. C¥172,800
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Answer: A. ¥76,800

With pay-as-you-go pricing, the charge is number of machines × hours run × unit price. The 2 regular machines run 2 × 24 × 30 = 1,440 hours and the 4 added machines 4 × 6 × 20 = 480 hours, 1,920 hours in total. The charge is 1,920 × 40 = ¥76,800. Running 6 machines all the time to cover the peak would cost 6 × 720 × 40 = ¥172,800, so adjusting the number of machines to the load keeps the cost below half.

  • A. Correct. (1,440 + 480) × 40 = ¥76,800.
  • B. This counts the 4 added machines on all 30 days instead of the 20 weekdays ((1,440 + 4 × 6 × 30) × 40 = ¥86,400).
  • C. This is the value for running 6 machines all month (6 × 720 × 40).
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