The National Council of Teachers of Mathematics (NCTM) in the US defended how the paper was marked, saying it gives students a better understanding of the problems they are solving.
"Part of what we are trying to teach children is to become problem solvers and thinkers," said Diane Briars, president of the NCTM.
"We want students to understand what they're doing, not just get the right answer."
The Common Core State Standards for English and Maths has been widely adopted in the US and outlines what each student should know in each subject by the end of each grade.
Critics claim the different approach can be too confusing for children, while supporters believe the methodology pays dividends when students move on to tackle more difficult problems.
"I would totally argue with the teacher over that for my child," commented one user.
However another replied: "This is a mark of a good teacher. If your question doesn't achieve the desired result then the question was the problem, not the answer."
The maths question has been viewed by more than 2.9 million people on Imgur.
