170 THE FASCINATION OF NUMBERS

Wednesday, for example, is 2 days later than the preceding
Monday. Every Monday is 7 days later than the preced-
ing Monday. If the date of the first Monday in any month
be the 1st, then other dates are calculated:

M. ST W,
I 2 3
8 9 IO
or M. A W.
A1 141 1+2 B

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and it is obvious that
A+D=B+C
Just as there is a fixed relationship between the first day
and any other day of a month, so there is also a similar
though more complex relationship between the first day and
any other day of a complete century. Also, in the same way as
we can calculate the date of any day, so may we perform the
reverse operation of ascertaining on what day of the week
any date will fall.
If we confine ourselves to the twentieth century beginning
1900 we may proceed as follows for any date. Add together:
(a) The last two digits of the year.
(b) A quarter of this same figure (ignoring fractions).
(¢) The code number for the month (see below).
(d) The number of the day in the month.
The code numbers for the months January to December are,
in order,

05 30 8 Bi Iy ! 0920 50 10 By B
Therefore, to find the day on which the 25th December 1954
fell, we have:

(@) 54
(b) 13
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(d) 25

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