Add up 1 to 100

And the answer was:

The candidate said: "Of course they are not expecting you to add it up. I realised you can pair up the numbers, (1+100), (2+99) etc. There were 50 pairs so 101 * 50 = 5050."

Additional information

This was asked for a derivatives middle office role. The candidate was not offered the job, which wasn't deemed testing enough for him.