5x3 may not look like much of a puzzle but this particular maths question may just be the most challenging one you come across today, if parents’ online reactions are anything to go by.

According to one post doing the rounds on image sharing site, Imgur, a US third-grade school pupil correctly answered the question ‘15’ - as expected - only for the teacher to mark it incorrect as part of the country’s controversial Common Core standards.

Having highlighted the solution behind the question as 5+5+5, the correct answer, in fact, should have shown five groups of three instead.

A second question in the paper, 4x6 - which was also answered correctly - was marked down because of what the teacher saw as being incorrect working, as highlighted:

The Common Core State Standards Initiative is an American educational initiative which outlines what first to 12th graders should know in English and mathematics at the end of every grade.

The image, which was posted on Imgur one week ago, has, so far, received almost three million views with many taking to the site to blast the views of the ‘pettiness’ of the teacher's marking.

One user suggested the teacher go back to school while another wrote: “There aren't enough curse words in the English language for me to express how I feel about this.”

Despite many criticising the standards the Common Core looks for, US business news site, Business Insider, reported how its defenders have said such methods will be useful when students eventually move on to do more advanced maths, including more challenging calculus problems which are taught in US high schools.

According to Tech Insider, high school maths and physics teacher Frank Noschese from New York described how the standards ‘just lay out what kids should know and be able to do, not actual lessons’.