This question from a global maths Olympiad for high school children is driving the internet crazy.
It was posted on Facebook by Singaporean TV host Kenneth Kong after the exam on Wednesday. He originally suggested it was for 11-year-olds, but after the question was leaked The Straits Times reports it's actually designed for the Australian equivalent of Year 9 and 10 students.
The leak has led organisers of the global competition to hastily rewrite the exam paper with a new question before other countries sit the test, with Singapore and Asian Schools Math Olympiad executive director Henry Ong saying it's the first time a question has leaked in 10 years of running the competition.
Ong told Singapore's Mothership that it was the 24th of 25 questions and designed "to sift out the better students."
The brain teaser has gone viral out Facebook and we've put the answer below, but if you want to know how the answer is calculated, Ong gave Mothership the solution. It's not so much maths as a deductive process of elimination.
And if you want to test your mathematical prowess against kids aged between 8 and 16, then SASMO has practice exam papers here.
Perhaps the best answer to the question came from KinMun Theodore Lee on Facebook, when he said regardless of the date, "It doesn't change the fact that Cheryl is going to be celebrating her birthday alone."
* The correct answer is July 16.
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