Last month, British student Alex Chui became the all-time most successful contestant at the International Mathematical Olympiad.

Chui, a year 13 student at Tonbridge School in Kent, scored 100 per cent in this year’s event, held in Shanghai, winning him a gold medal, and bringing his tally of medals to seven, five of them golds.

No other contestant in the history of the IMO, which has been running since 1959 and brings together school-age contestants from more than a hundred countries, has won a medal seven years in a row.

I asked Alex if he had a favourite puzzle, and he suggested the following gem.

How many ways are there to fill a 3×3 grid with positive whole numbers such that the product of the numbers in each row and each column is 30?

[Each of the nine cells in the grid has a whole number in it. When you multiply the three numbers in each row, and in every column, the answer is 30. The three numbers in each row or column do not have to be three different numbers.]

It’s a very elegant puzzle. Your first instinct might be to try to count the completed grids individually. But that will take you a while, since it can be done in more than 200 ways.

I will reveal his solution to you at 5pm UK today.

PLEASE NO SPOILERS Instead discuss grids.

UPDATE: Solution is here.

The IMO takes place every year, and is held over two days. Each day is a four and a half hour exam with three questions, each worth seven points. This year is the first that Chui – who will study maths at Cambridge next term – scored 42/42.

I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.