Friday 13th is a sentimental rather than a worrying day for me. It reminds me of my Grandad staying indoors on “Black Friday” for fear of some dreadful event occuring. As a child I linked his strong belief in luck to his years at sea. He told a dramatic story of how he had decided at the last minute not to take work on a wonderful new ship, the Titanic.

My Grandad’s dislike of the number also meant that he refused to travel in a car if the digits on the number plate added up to 13. I’ve often used this as a starting point for mental arithmetic. Deciding whether or not the digits on any number plate total 13 is a relatively simple activity. An extension is to try to find all the possible combinations of three single digit numbers that total 13. Variations include using number plates with different numbers of digits by considering foreign number plates, or examining “unlucky” phone numbers.

Other “Black Friday” maths can be found by looking at calendars, which offer opportunities both for calculating and pattern spotting. Now if you are really nervous about Friday the 13th, you can always blot it or cut it out of your calendar. On a grid-style calendar, however, you will find that the numbers above and below the offending number add up to double 13, as do those either side of it and the pairs of numbers touching it diagonally. Alternatively, if you use a calendar where the dates are listed vertically and blot out 13, the 12 numbers above it will still add up to a multiple of 13. This might initially seem like black magic, but children may realise that the numbers can be paired to total 13 (1+12, 2+11, 3+10, etc.) at which point the magic of mathematics is revealed.

To take a really cautious approach to the number 13, you could avoid using it altogether. Some hotels favour a floor 12b, so you could refer to the Friday in question as 12+1 or 7+6. Finding pairs that total 13 can be followed by asking for particular types of number. For example, can the children make 13 by adding two prime numbers or two square numbers? Can they explain why it is impossible to reach this total with two odd numbers or two even numbers? The activity can be extended to use subtraction and combinations of operations or fractions or percentages. Children accustomed to the activity can delight in short cuts. But I’m happy to see the answer “half of 26” followed by “50 per cent of 26” or “26 x 0.5” as these children may be saving themselves some calculation, but are also showing a good understanding of the connections between different aspects of number.

More difficult calculations involve the time lag between one Friday 13th and the next, which is complicated by the differing lengths of months. In 1998, however, we have two “Black Fridays” only one month apart.

* `BLACK FRIDAY’ MATHS PUZZLES does not appear on this database