A sheet of plywood is 5/8 inches thick. How tall is the stack of 21 sheets?

To find the height of the stack of 21 sheets of plywood, we need to multiply the thickness of one sheet by the number of sheets.

The thickness of one sheet is 5/8 inches.
So, the height of the stack of 21 sheets is (5/8) * 21 = 5 * 21/8 = 105/8 = 13 1/8 inches. Answer: \boxed{13\frac{1}{8}}\text{ inches}.