If a big brother is four years old and the little brother is half his age, how old would the little brother be when the big brother is 100?
That's the simple maths question which has perplexed one woman so much that it's led to her becoming something of an internet sensation.
The clip above shows YouTube user 'xmintyx' testing his girlfriend's numeracy skills.
Firstly, she repeats the question above, before calmly answering: "He'll be 50."
Her boyfriend then laughs at her and asks her the question again, while she sits there looking incredibly puzzled. She repeats her first answer.
After her boyfriend realises that she isn't going to come to the correct conclusion any time soon, he calmly reveals that the younger brother will be 98.
To which his girlfriend replies: "That doesn't make sense though. I think you've got this wrong."
And then the penny drops: "Oh, I get it."
The hilarious clip has been viewed more than one million times. It has, as could be expected, sent the comments section on YouTube into overdrive.
One user wrote: "I still don't get it... Someone explain please ."
While another added: "I felt really stupid because I didn't get the joke even after the video ended... Then I asked my mom and she immediately said 98."
Ah the joys of the internet.