Most powerful test between two discrete uniforms

Let $X$ be a random sample from a discrete distribution with the probability mass function

$$f(x;\theta)=P(X=x)=\begin{cases}\dfrac1\theta, & x=1,2,…,\theta,\ 0, & \text{otherwise,}\end{cases}$$

where $\theta\in\Theta=\{20,40\}$ is the unknown parameter. Consider testing $H_0:\theta=40$ against $H_1:\theta=20$ at a level of significance $\alpha=0.1$. Then the uniformly most powerful test rejects $H_0$ if and only if