A clock face is a circle: 360° divided into 12 hours. Each hour mark is 30°. Each minute is 6° (because 360 ÷ 60 = 6).
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Angle between hands at 3:00
Hour hand at 3, minute hand at 12. That's 3 hours apart × 30° = 90°. A right angle.
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Angle at 4:00
4 hours × 30° = 120°. Obtuse.
At 6:00 the hands are 180° apart — a straight line. At 12:00 they overlap (0°).