My sister Mathy used to live at Holm East St., a long street with 999 door numbers. When I tell her that I want to visit her, she said:
- OK. But remember that I have moved. I am not anymore at number 647 but I have moved ahead, in the same street, to a completely different prime number.
- What is it?, I asked.
- Oh!, you should guess it, she said. If you sum the digits of my new number it happens that there are just 647 doors with an inferior sum.
So, where does she live now?