I think "looking at the clock" is meant to suggest using modular arithmetic. I have heard modular arithmetic explained that way before (i.e. for an analog clock, the hour is incremented modulo 12).

btw, I think the answer is unique.

## Finding the nine digit number

I think "looking at the clock" is meant to suggest using modular arithmetic. I have heard modular arithmetic explained that way before (i.e. for an analog clock, the hour is incremented modulo 12).

btw, I think the answer is unique.