If I have 30-15" square blocks. If I cut strips 21/2" wide & sew them on blocks what size will the quilt be?

If you sew the blocks together and only put the 2 1/2" strip on the outside, and uming you are using the traditional 1/4" seam allowance, the quilt should be 76.5" x 91" (that's 5 blocks wide and 6 blocks long). If you use the 2 1/2" as sashing between all the blocks and on the outside of the blocks, the finished quilt size will be 84.5" x 101". Remember, you are using 1/2" of each block as a seam allowance, so the finished block will be 14.5" wide and the strips will be 2" wide when finished sewing. I hope that helps.