Man pranks his girlfriend with a maths riddle that she fails to get right in an embarrassing viral video (but can YOU answer the question?)

UK YouTube user xmintyx's amusing clip showing him repeatedly ask his baffled girlfriend a trick maths question has had more than one million views online. 

She is adamant she has the answer right as her partner talks her through the brain teaser several times. 

'The big brother is four years old. The little brother is half his age so that makes him two. How old would the big brother be when he's 100? He'll be 50,' she says, with confidence. 

While many branded the clip 'humiliating', others confessed they were left equally stumped by the riddle.