How many 15.75" square blocks are in a 16' x 18 ' plot?

The easiest way to work it out is to convert the plot to inches.

16' x 18' = 192" x 216"

The divide each side by 15.75" and ditch the remainder.

192" / 15.75" = 12 (with a bit left over)

216" / 15.75" = 13 (with quite a bit left over)

In terms of square blocks, you get:

12 x 13

Or 156 blocks.