Brain Teasers
Clock Hands II
A
pocket watch is placed next to a digital clock. Several times a day, the
product of the hours and minutes on the digital clock is equal to the number of
degrees between the hands of the watch. (The watch does not have a second
hand.) As you can see, 10:27 is not one of those times — the angle between
the hands is not 270°. If fractional minutes aren’t allowed, find the
times at which the product of the hours and minutes is equal to the number of
degrees between the hands.
This brainteaser was
written by Derrick Niederman.
Solution:
3:36 and 11:20.
The
angle between the hour and minute hands equals the product of the hours and
minutes at 3:36 and 11.20. In the latter case, the product equals 220,
which is the number of degrees between the hands—if you go the long way around!
Let
h = hours and m = minutes. The hour hand covers 30° every hour, so at the top of
any hour it has traveled 30h degrees,
and with every passing minute it travels another m/2 degrees. Given that the minute hand moves 6° every minute,
the number of degrees between the hands is given by either 6m – (30h + m/2) or (30h + m/2) – 6m. Consequently,
the relevant equations are:
Because m is a positive integer, the
first equation implies that 11 – 2h
must divide evenly into 60h, and that
happens only when h = 3.
Plugging in h = 3 yields m = 36, so (3, 36), or 3:36,
must have the desired property. Sure enough, there are precisely 3 × 36 = 108°
between the hour and minute hands at 3:36.
For
the second equation, the right-hand side must be a positive integer, and that
happens only when h = 11. In that
instance m = 20, producing (11, 20),
or 11:20, as our second time. The fact that you go the “long way around”
in this instance arises because the second equation essentially reverses the
order of the hour and minute hands.
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