Maths comes naturally to some, but even simple equations can become baffling brainteasers. Here are 10 problems that confused people across the internet.
One viral puzzle, originally from Go Tumble and shared on Wikr before spreading on Facebook, has two correct answers: 40 or 96. The first method involves adding the equation and combining the sum with the previous result; the second multiplies the second number by the number being added.
Another problem sparked debate over order of operations (PEMDAS/BODMAS). The modern interpretation gives an answer of 9, but the historical rule from before 1917 yields 1—a version still taught in some schools.
A controversial homework question asked for 5 x 3 using repeated addition. A student wrote 5+5+5=15 but was marked wrong; the teacher insisted on 3+3+3+3+3=15. Similarly, for 4 x 6 using an array, the student's six-row, four-column array was rejected in favour of four rows and six columns, despite the commutative property of multiplication.
A logic puzzle from Singapore's SASMO contest, shared by TV host Kenneth Kong, asked for Cheryl's birthday based on clues about day and month. Using elimination, the answer is 16 July. Another problem, tweeted by a UK mum, involved a train with 63 passengers after 19 got off and 17 got on; the initial number was 65.
A picture puzzle showed numbers 86 to 91 in a car park; turning the image upside down reveals the sequence. A 'missing dollar' riddle was debunked by focusing on total debt rather than misleading wording. A third-grade puzzle required placing digits 1-9 in a snake equation to equal 66, solved by trial and error. Finally, a bat and ball problem: a bat costing £1 more than a ball, total £1.10, means the ball costs 5p.
