What are real numbers for this rational expression: x^3-8/5x-20???

Answer 1

If you mean

(x^3 - 8)/(5x - 20)

then you get real numbers as long as the expression exists. The expression doesn't exist when the denominator is 0 - that would make the expression undefined. So,

5x - 20 = 0
5x = 20
x = 4

So you would have real numbers for all x except for 4.

Answer 2

Real numbers are 8/5, 20. x^3 is a variable.