# Enigmatic Code

Programming Enigma Puzzles

## Enigma 532: Friday the 13th

From New Scientist #1684, 30th September 1989 [link]

John Thomas, who is 50 this year [i.e. in 1989], was not really superstitious till Betty left him. In fact, they were engaged on a Friday the 13th. They were married after 13 weeks of courtship on a Friday the 13th. The married couple were abroad for their Christmas vacation. Towards the end of that vacation they quarrelled and disagreed on almost everything. They were eventually separated on a Friday the 13th, after exactly 13 weeks of married life. These events made John very, very superstitious.

When did Betty leave John?

[enigma532]

### 2 responses to “Enigma 532: Friday the 13th”

1. Jim Randell 30 December 2019 at 8:39 am

This Python program looks for suitable dates. It runs in 92ms.

Run: [ @repl.it ]

```import datetime
from enigma import printf

# consider dates from 1939 - 1989
d = datetime.date(1939, 1, 1)
i = datetime.timedelta(days=1)

# collect dates that fall on Friday 13th
ds = list()
while d.year < 1990:
if d.day == 13 and d.isoweekday() == 5:
ds.append(d)
d += i

# look for three dates separated by 13 weeks
i = datetime.timedelta(weeks=13)
for d1 in ds:
d2 = d1 + i
d3 = d2 + i
if not(d2 in ds and d3 in ds): continue

# d2 is before Christmas, and d3 is after it
if not(d2 < datetime.date(day=25, month=12, year=d2.year) < d3): continue

printf("engaged = {d1}; married = {d2}; divorced = {d3}")
```

Solution: They separated on 13th March 1964.

The were engaged on 13th September 1963, and married on 13th December 1963.

2. Hugh Casement 30 December 2019 at 9:51 am

From the nth of September to the nth of December is 30 + 31 + 30 days = 13 weeks.
If the following year is a leap year, then it’s 31 + 31 + 29 days to the nth of March.

The sequence had previously occurred in 1935-36 (before John was born)
and occurred again in 1991-92 (after the puzzle was set).
Also 2019-20, but in that case they’re still quarrelling and she hasn’t left him yet.

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