Significance level of a drug efficacy test on 20 patients

A drugmaker says its treatment improves focus in $80\%$ of patients with attention deficit disorder. A regulator suspects this is too high, gives the drug to $20$ patients, and counts the number $X$ who respond, testing $H_0:p=0.8$ against $H_a:p<0.8$ and rejecting when $X\le12$. What is the significance level $\alpha$?

About $0.032$. The significance level is the chance of landing in the rejection region when $H_0$ holds: $\mathbb{P}(X\le12)$ for $X\sim\text{Bin}(20,0.8)$, which is approximately $0.0321$.