Solution for #47

Suzy's birthday must be on December 31 of some year. Let's say she was born on December 31, 2000. That means Suzy's ninth birthday is December 31, 2009. And let's say "today" is January 1, 2010.

If we suppose these things, then two days earlier would be December 30, 2009, the day before Suzy's ninth birthday. She was 8.

Now let's get back to today's date, January 1, 2010. "Next year" is 2011. On January 1, 2011, she'll only be 10, but on December 31 that same year, she'll be 11.