What's the square root of 9876?

"Mental calculator' Dr Amit Garg can answer that question faster than you can type it into your calculator.

The answer is 99.4 (to one decimal place) and it came almost instantly from Dr Garg, when we meet at Auckland's AUT.

"I have a few algorithms which I use because I am both a mathematician and mental calculator," says Dr Garg.

"Initially I figured out how to do the square root for a two digit number in the most efficient manner, but now I can do it for numbers with many digits."

Dr Garg is a Guinness World Record holder in mathematics.

In 2012, he broke the mental calculation world record of completing ten tasks to "divide a 10-digit number by a 5-digit number" in 5:45 minutes without error.

"My mathematical skills already had been strong from the age of three because I had a lot of interest in the numbers.

"When I started studying engineer I felt it took me less time to calculate myself rather than do it on the calculator."

Dr Garg is now founder and chief scientist at ORMAE, a company based in India developing algorithms for Operations Research, Machine Learning and Analytics.

The algorithms he develops can be used in everything from working out the optimal route to spray agricultural chemicals in a 2,000-square-kilometre area by tractor to shift timings so call centre agents can answer calls within 20 seconds with the least number of employees on duty.

"There are many situations where a computer or calculator cannot help me, so I prefer to do it in my head," says Dr Garg.

This includes the extraordinary feat of calculating the square root of numbers with more than 100 digits – something Dr Garg says only 5 or 6 people in the world are capable of doing.

But while his mental calculation abilities are beyond comprehension for most people, Dr Garg does admit he has to be careful not to overdo it.

"There is a limit of about 45 minutes of high level calculations because your brain heats up too much.

"You have to wait for a few hours for the brain to cool down before you can start calculating again."

So what's the square root of 195, 065?

It's 441.7 and Dr Garg got it in just 5 seconds.