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KCC's Quizzes AQQ230 about a clock and angle puzzle

In an analog clock, the angle formed by the hour and the minute hands varies constantly.

Questions:

  1. Which angle θ is obtained when it is 5:20 ?
  2. Determine when exactly at around 4, the angle will be exactly 90°?


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[edited by: emassa at 1:54 PM (GMT -4) on 29 Mar 2023]
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  • 1. Which angle θ is obtained when it is 5:20 ?

    Long hand: 20 past is 120 degrees from 12:00
    Short hand: is 150 degrees + 1/3 of the way between 5 and 6 (30/3 degrees) = 160 degrees
    Angle between them is 40 degrees

    2. Determine when exactly at around 4, the angle will be exactly 90°?

    I started iterating it...

    4:00pm
    Long hand: 0
    Short Hand: 120
    Difference: 120

    4:10pm
    Long hand: 60
    Short Hand: 120 + 30/6 = 5
    Difference: 65

    4:05pm
    Long hand: 5/60*360 = 30
    Short Hand: 120 + 30*5/60 = 2.5
    Difference: 92.5 Too high - add time

    4:06pm
    Long hand: 6/60*360 = 36
    Short Hand: 120 + 30*6/60 = 3
    Difference: 87 Too low - subtract time

    At this point we know it is between 4:05 and 4:06.  I iterated some more but this successive approximation was a little painful, so I thought a bit more and came up with the equation I was essentially using.

    l=long hand angle
    s=short hand angle
    m=minutes past

    s-l = 90
    l=6*m
    s=120 +(30*m/60)
    So
    120 + (0.5*m) - 6*m = 90
    -5.5*m = -30
    m = 30/5.5 = 5.45 minutes past 4 or 4:05:27

Reply
  • 1. Which angle θ is obtained when it is 5:20 ?

    Long hand: 20 past is 120 degrees from 12:00
    Short hand: is 150 degrees + 1/3 of the way between 5 and 6 (30/3 degrees) = 160 degrees
    Angle between them is 40 degrees

    2. Determine when exactly at around 4, the angle will be exactly 90°?

    I started iterating it...

    4:00pm
    Long hand: 0
    Short Hand: 120
    Difference: 120

    4:10pm
    Long hand: 60
    Short Hand: 120 + 30/6 = 5
    Difference: 65

    4:05pm
    Long hand: 5/60*360 = 30
    Short Hand: 120 + 30*5/60 = 2.5
    Difference: 92.5 Too high - add time

    4:06pm
    Long hand: 6/60*360 = 36
    Short Hand: 120 + 30*6/60 = 3
    Difference: 87 Too low - subtract time

    At this point we know it is between 4:05 and 4:06.  I iterated some more but this successive approximation was a little painful, so I thought a bit more and came up with the equation I was essentially using.

    l=long hand angle
    s=short hand angle
    m=minutes past

    s-l = 90
    l=6*m
    s=120 +(30*m/60)
    So
    120 + (0.5*m) - 6*m = 90
    -5.5*m = -30
    m = 30/5.5 = 5.45 minutes past 4 or 4:05:27

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