Xiao Xin uses some hard forks to form a square solid phalanx, with 7 pieces on each side of the outermost layer. How many pieces of this square are hard?

There are seven coins on each side, and a square has four sides. It should be four or 728 coins. But the coins on the four corners of the square are vertical and horizontal and must be subtracted, so the coins on this square are:

4*7-4=24 coins.

I hope it can help you solve the problem.