What are the possible rational zeros of f(x)=x^4+2x^3-3x^2-4x+20?

Roots test tells us to take the factors of the 20 divided by the factors of the coefficient of the the first.  

factors of 20 are 1,2,4,5,10,20
factors of 1 are 1

so plus or minus 1/1, 2/1, 4/1, 5/1, 10/1, 20/1 are all possible rational zeros