In order to find if a number is a perfect square, one needs to factor it out and see what its factors look like.
In our case, when we factor out 40 extracting common factors ione at a time, we get:
40 = 20 * 2 = 10 * 2 * 2 = 5 * 2 * 2 * 2
This can be written as
which is NOT an expression to the square power.
So this number is NOT a perfect square.
81 is a rational number, because it can be written as a "quotient" (ratio) of integers in the form: 81/1