A rectangular floor is covered with 315 black and red tiles. There is a border of single black tiles round the edge of the floor, and inside that a pattern of 2 interlocking black squares which do not touch the border in any place. Some of the tiles will need to be cut in half along the diagonal.

The sides of these black squares are 6 tiles long, set diagonally.

How many of the 315 tiles are black?