Not exactly a maths puzzle, but does need working out,
"When the day after tomorrow is yesterday", "then 'today' will be as far from Sunday as that day which was 'today' when the day before yesterday was tomorrow!"
You start to write the natural number sequence: 0, 1, 2, 3, 4, 5, 6, 7, ...... What number will you be writing when you write the digit 7 for the 700th time?