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What is the Time? (Posted on 2023-09-12) Difficulty: 3 of 5
The two hands of a clock are so situated that, reckoning as minute points past XII, one is exactly the square of the distance of the other.

Determine all possible times that conform to the given conditions.

**** Adapted from an original problem by H. E. Dudeney.

See The Solution Submitted by K Sengupta    
Rating: 5.0000 (1 votes)

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Solution computer-aided solution | Comment 1 of 11
We're measuring angle clockwise from the top so that one revolution equals 60 units, so if the position of the minute hand is to be the square of the position of the hour hand, it cannot be as much as 2 PM, as even at 2 PM, the square of 10 is 100 -- outside the range of a clocface.  In every hour a time can be reached where the minute hand position squared equals the hour hand position.

syms m
for h=0:11
  hh=5*h+m/60;
  mh=m;
  % 1st: hh^2 = mh
  % m=(5*h+m/60)^2 = 25*h^2 + m^2/3600 + h*m/6
  % m^2/3600 + h*m/6 - m + 25*h^2 = 0
  disp(h)
  s=roots([1/3600,h/6-1,25*h^2]);
  disp([s,5*h+s/60])
  %then min hand is sqrt of hour hand
  s=roots([1 -1/60 -5*h]);
  disp([s,5*h+s/60])
  disp('--------')
end

solves the quadratic equations, including spurious results, so we have to throw out 3 out of 4 equation solutions:

>> whatIsTheTime
     0
           0           0
        3600          60
                         0                         0
        0.0166666666666667      0.000277777777777778
--------
     1
          2969.69384566991          54.4948974278318
          30.3061543300931          5.50510257216822
          2.24441683902905          5.03740694731715
         -2.22775017236238          4.96287083046063
--------
     2
          2239.23048454133          47.3205080756888
          160.769515458674          12.6794919243112
          3.17062197361341          10.0528436995602
         -3.15395530694675          9.94743407821755
--------
     3
                       900                        30
                       900                        30
           3.8813256447696          15.0646887607462
         -3.86465897810293          14.9355890170316
--------
     4
  Column 1
                        600 +      1039.23048454133i
                        600 -      1039.23048454133i
  Column 2
                         30 +      17.3205080756888i
                         30 -      17.3205080756888i
           4.4804770524511          20.0746746175409
         -4.46381038578443          19.9256031602369
--------
     5
  Column 1
                        300 +      1469.69384566991i
                        300 -      1469.69384566991i
  Column 2
                         30 +      24.4948974278318i
                         30 -      24.4948974278318i
          5.00834027777296          25.0834723379629
         -4.99167361110629          24.9168054398149
--------
     6
  Column 1
                          0 +                  1800i
                          0 -                  1800i
  Column 2
                         30 +                    30i
                         30 -                    30i
          5.48556524776278          30.0914260874627
         -5.46889858109611          29.9088516903151
--------
     7
  Column 1
                       -300 +      2078.46096908265i
                       -300 -      2078.46096908265i
  Column 2
                         30 +      34.6410161513775i
                         30 -      34.6410161513775i
          5.92441898555681          35.0987403164259
         -5.90775231889014          34.9015374613518
--------
     8
  Column 1
                       -600 +      2323.79000772445i
                       -600 -      2323.79000772445i
  Column 2
                         30 +      38.7298334620742i
                         30 -      38.7298334620742i
          6.33289414373309          40.1055482357289
         -6.31622747706642          39.8947295420489
--------
     9
  Column 1
                       -900 +      2545.58441227157i
                       -900 -      2545.58441227157i
  Column 2
                         30 +      42.4264068711929i
                         30 -      42.4264068711929i
          6.71654244191399          45.1119423740319
         -6.69987577524732          44.8883354037459
--------
    10
  Column 1
                      -1200 +       2749.5454169735i
                      -1200 -       2749.5454169735i
  Column 2
                         30 +      45.8257569495584i
                         30 -      45.8257569495584i
          7.07940605566086          50.1179901009277
         -7.06273938899419          49.8822876768501
--------
    11
  Column 1
                      -1500 +      2939.38769133981i
                      -1500 -      2939.38769133981i
  Column 2
                         30 +      48.9897948556636i
                         30 -      48.9897948556636i
          7.42453650237101          55.1237422750395
         -7.40786983570434          54.8765355027383
--------

At 12:00 exactly, both hands are at 0, and 0^2 = 0.

At 1:02.24441683902905 the minute hand is at the minute portion shown, and the hour hand is at 5.03740694731715, the square of the minute hand position.

At 1:30.3061543300931 the hour hand is at the square root of that minute hand position: 5.50510257216822. That's the last time that the minute hand will the at the square of the hour hand.

At 2:03.17062197361341 the hour hand will be at 10.0528436995602.

At 3:03.8813256447696  the hour hand will be at 15.0646887607462.

At 4:04.4804770524511  the hour hand will be at 20.0746746175409.

At 5:05.00834027777296 the hour hand will be at 25.0834723379629.

At 6:05.48556524776278 the hour hand will be at 30.0914260874627.

At 7:05.92441898555681 the hour hand will be at 35.0987403164259.

At 8:06.33289414373309 the hour hand will be at 40.1055482357289.

At 9:06.71654244191399 the hour hand will be at 45.1119423740319.

At 10:07.07940605566086 the hour hand will be at 50.1179901009277.

At 11:07.42453650237101 the hour hand will be at 55.1237422750395.

 

  Posted by Charlie on 2023-09-12 09:07:15
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