Question 143952
The first square had 24 left over pieces.
Adding a row and column, puts 12 across one row and 13 in the column (since the corner piece is part of both the row and column).

We are told that configuration leaves us 25 pieces short of a square. We will need 13 pieces to compete the row and 12 pieces to complete the column. At that point, we would have a square that is 25 on each side.

So he has {{{24*24 + 24 }}} or {{{25*25 - 25 }}} = 600