Happy to meet you. I was there and I remember that question being asked. I think it was 2010.

If I remember correctly he had two immediate ideas, his first was bubble sort, the second turned out to be quicksort.

He was already very frail by then. Yet clarity of mind was undiminished. What came across in that talk, in addition to his technical material, was his humor and warmth.

I remember this vividly! I believe he said that he thought of _Bubble Sort_ first, but that it was too slow, so he came up with QuickSort next

He discusses this and his sixpence wager here: https://youtu.be/pJgKYn0lcno

(Source: TFA)