HOW many ways are there to place four marbles of different colours into four empty boxes?
More Singapore students are cracking their brains over mathematics questions like this and the one dubbed "Cheryl's Birthday" which left adults from Britain to Canada scratching their heads.
And students here are getting better at solving these puzzles.
Enrichment centres are training more students in Olympiad concepts such as combinatorics - the study of counting and permutations - and teaching them tricks such as how to tell if a number can be divided by other numbers.
One of the reasons for the growing interest in maths questions outside the curriculum could be because Maths Olympiad scores can be used for direct admission into secondary schools, said some trainers.
More than 9,000 students took part in the Singapore Mathematical Olympiad last year, up from around 7,000 a decade ago.
The contest is run by the Singapore Mathematical Society (SMS) for secondary and junior college students, and the top six represent Singapore at the International Mathematical Olympiad (IMO).
Singapore has improved at the IMO, held annually by different countries.
It first took part in the event in 1988 and was placed 15th to 40th in the early years. Its best showing was third in 2011, and it has been in the top 10 every year since. Last year, it was eighth.
Separately, the NUS High School of Mathematics and Science has been running the National Mathematical Olympiad of Singapore for Primary 5 pupils since 2006. About 5,000 pupils now take part in it, up from 2,500 when it first started.
Schools that hold maths contests for primary school pupils, such as Anglo-Chinese School (Independent) and Raffles Institution (RI), also reported increased participation.
Mr Michael Lim, founder of Maths Hub, an enrichment centre, said it trains more than 1,000 students in schools and at five centres, up from just 110 in 2004.
Mr Henry Ong, a manager for Excel League, which publishes Olympiad materials and runs competitions, said it trained students in seven schools last year, up from two in 2002.
SMS vice-president Victor Tan said: "Some students are genuinely interested in maths and want to test their ability, but many do it for practical reasons."
Referring to using Olympiad results to gain direct admission to schools, he added: "Many students get As for maths, so they need to distinguish themselves from the rest."
RI Year 6 student Tan Siah Yong, 18, who represented Singapore at the IMO in the last two years, said training for the contest has exposed him to a variety of problem sums and creative thinking. The Gifted Education Programme student said: "It's not just about calculating something."
Primary 3 pupil Siddhaarth Dharani, nine, of North Spring Primary, who took part in three contests this year, said: "I like maths as it's fun and challenging. I also like science because concepts in maths explain patterns in science."
And in case you were wondering, the answer to the question at the start is: 15.