Minimum Angle Between Clock Hands

XAT 2021 · Quantitative Ability & Data Interpretation · Medium · Clocks

At any point of time, let xx be the smaller of the two angles made by the hour hand with the minute hand on an analogue clock (in degrees). During the time interval from 2:30 p.m. to 3:00 p.m., what is the minimum possible value of xx?

  1. A.

    45

  2. B.

    105

  3. C.

    90

  4. D.

    0

  5. E.

    75

Answer

C

Explanation

Angle between hands at time H:MH:M is given by ∣30H−5.5M∣|30H - 5.5M|. At 2:30 p.m.: ∣30(2)−5.5(30)∣=∣60−165∣=105∘|30(2) - 5.5(30)| = |60 - 165| = 105^\circ. As minute hand moves from 30 to 60, the hour hand moves from 2.5 to 3. At MM minutes past 2 p.m., angle is ∣60+0.5M−6M∣=∣60−5.5M∣|60 + 0.5M - 6M| = |60 - 5.5M|. For M∈[30,60]M \in [30, 60], 60−5.5M60 - 5.5M is strictly negative and decreasing (its absolute value increases from 105 at M=30M=30 to 270 at M=60M=60). So smaller angle x=360∘−(5.5M−60)x = 360^\circ - (5.5M - 60). At 3:00 p.m., the angle is 90∘90^\circ. Throughout 2:302:30 to 3:003:00, the angle continuously decreases to a minimum of 90∘90^\circ at 3:00 p.m.

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