A thermometer measures temperature in whole degrees only. If the temperature falls below 18 degrees, the heating is switched off. It is switched on again when the temperature reaches 21 degrees…

Question

Options

• A15, 19 and 25.
• B17, 18, and 19.
• C18, 20 and 22.
• D16, 26, and 32.

• A
  77% (46)
• B
  13% (8)
• C
  3% (2)
• D
  7% (4)

Explanation

Option A (15, 19, 25) correctly represents all three equivalence partitions: below 18 (heating off), 18–20 (neither trigger), and 21 and above (heating on) - with 15, 19, and 25 each landing in exactly one of those zones.

Option B (17, 18, 19) fails because both 18 and 19 fall in the same middle partition (18–20); the "heating on" partition (≥21) is never tested. Option C (18, 20, 22) misses the "below 18" partition entirely, as 18 and 20 are both in the middle zone. Option D (16, 26, 32) skips the middle partition (18–20) altogether, with 26 and 32 redundantly testing the same "heating on" zone.

Memory tip: Equivalence partitioning means one value per zone, all zones covered - if two of your three values land in the same zone, you've wasted a test case and left a partition untested.

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