Aconcrete stepping stone measures 20 inches square. what is the area of 30 such tiles?

A single stepping stone as you have said is 20 inches sqaure. This means that it has an area of 20.

All you would need to do then is multiply 20 by 30 to get the area of thirty tiles. The result of this would be 600.

If you mean one side of the stepping stone is 20 inches then you would multiply 20 by 20 to get the area of one, since that would be multiplying length times width (the area formula). The area of one would result in 400 and you would then multiply this by the thirty tiles (400 times 30) to get 12000.