Railways Notes / Mathematics (Quantitative Aptitude) / Chapter 29
Chapter 29 of 31

Clock

The angle-between-hands formula and faulty clock problems — a compact, formula-driven chapter.

📖 ~4 min read 📐 RRB NTPC / Group D — Mathematics

Key Facts

  • The minute hand moves 6° per minute
  • The hour hand moves 0.5° per minute (30° per hour)
  • Every hour, both hands coincide once and are opposite (180°) once

Angle Between Hands Formula

Angle = |(30×H) − (11/2)×M| degrees, where H = hour, M = minutes
💡 Example: Angle at 4:20 → |(30×4) − (11/2)×20| = |120 − 110| = 10°

Faulty Clocks

  • If a clock gains/loses time, the "correct" 12-hour period is stretched or compressed proportionally — use ratio/unitary method to find the actual elapsed time.

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