Whiling away the hours on a long flight can be very tedious, and passengers have a variety of things that keep them occupied.

One of the strangest that we heard about was adding up the page numbers in a pamphlet that was in the seat pocket. One plus two equals three, three plus three equals six, and so on.

In case you are interested the total was 90, which confused the passenger in question, until they realised that there was a sheet missing.

What were the numbers of the two pages on the missing sheet?

The sequence of numbers 1, 3, 6 etc. are known as triangular numbers, which are very easily calculated.

The first triangular number greater than 90 is 91, but that can't work as one can't be the sum of two page numbers. In addition this is the 13th triangular number and there could not be an odd number of pages in the original pamphlet.

The next triangular number is 105, which is the 14th triangular number, so that bit is fine. And it is 15 greater than 90, which is the sum of seven and eight, the page numbers on the missing sheet.