probablity that Colin will pass the test if he guesses

In a multiple choice test that contains ten questions with each question having five possible answers, what is the probablity that Colin will pass the test if he merely guesses at each question?

How would you do this? I know the equation, and that P (probability is 1/5) but im not sure what x and n would be.

P(x)= nCx * P^x * q^ (n-x)

Thanks

IF N is the number required to pass then:
\(\displaystyle \L \sum\limits_{k = N}^{10} {\left( {\begin{array}{c}
{10} \\
k \\
\end{array}} \right)\left( {\frac{1}{5}} \right)^k \left( {\frac{4}{5}} \right)^{10 - k} } .\)