Is it a root, or not?

$\large -400{ x }^{ 5 }+2660{ x }^{ 4 }-3602{ x }^{ 3 }+1510{ x }^{ 2 }+18x-90$

The polynomial above has five rational roots. Suppose you guess a rational number which could possibly be a root (according to rational root theorem), what is the probability that it actually is a root?

If your answer is $\frac ab$ for coprime positive integers $a,b$ . Submit your answer as $a+b$ .