Hypothesis Tests · Superchallenge problem

To test $H_0$: $p = \frac12$ against $H_a$: $p > \frac12$ with $20$ coin tosses, you will reject when the number of heads is at least $k$, choosing the smallest $k$ whose rejection probability under $H_0$ is at most $0.05$. What is the ACTUAL significance level of this test? Give your answer to four decimal places.