Startup guru Paul Graham, who co-founded the seed capital firm Y Combinator, tweeted out this maths problem to his followers on Thursday.
What is the next number in this sequence? 16, 17, 18, 18, 20, 19, __. Choices: 20, 21, 22, 23.
— Paul Graham (@paulg) January 15, 2015
Graham said the maths problem was a question on the 11-plus exam, a test given to some students in their last year of primary school. The 11-plus, which used to be widely used throughout England, is now only used in some counties in the country.
The 11-plus serves as an admission tool for students who are 11 or 12 years old and entering secondary school (that’s where it gets its name from). It has four sections: English, mathematics, non-verbal reasoning, and verbal reasoning.
Followers quickly responded that the right answer is 22. If you’re stumped, here are some of Graham’s followers’ answers and their explanations.