The human calculator: 393 trillion answers - and he picks the right one in 70 seconds
When the answer is 2,407,899,893,032,210 you know the question is tough.
Not so tough, however, that Alexis Lemaire could not work it out in his head.
His challenge yesterday was to come up with the 13th root of a computer-generated 200-digit number.And, with 393 trillion possible answers to choose from, the PhD student made it almost look easy.
A mere 70.2 seconds later, he cracked it and officially became the world's fastest human calculator.
A slight frown and a stare of deep concentration had been the only sign the 27-year-old "mathlete" was doing anything more than running through the eight times table.
Appropriately, the Frenchman broke his previous world record of 72.4 seconds at the Science Museum in London, where he had a backdrop of Charles Babbage's 1840s Difference Engine No2, the first successful mechanical calculator.
For those in the know, 13th roots are a yardstick in mental arithmetic for mathletes determined to show ever greater feats of brainpower.
A 13th root is - if your maths is no longer at Mr Lemaire's level - a number that multiplied by itself 13 times matches the initial figure.
But this soon became too easy. The first time he tried a 200-digit challenge, it took him 40 minutes.
Since then, he has put himself through a mental training regime that has seen him repeatedly cut his time.
Cracking the answer is, apparently not all about maths, it also owes a lot to memory. Mr Lemaire, who is single, has memorised thousands of combinations of 13th root numbers.
"It's a bit like multiplication tables but with huge digits," he said. "It's a combination of techniques, partly memory and partly maths."
Asked to explain further, he would only say: "I won't give you my secret."
He did, however, agree to try the Daily Mail's 30-Second Challenge, and finished the advanced task in eight seconds.
It was a more than respectable performance - but, for a champion "13th rooter", it didn't seem that impressive.
Perhaps he was still tired after his world record.