Hypothesis Tests · Superchallenge problem

Five observations are drawn from the uniform distribution on $[0, \theta]$. To test $H_0\!: \theta = 1$ against $H_a\!: \theta < 1$, reject $H_0$ when the largest observation is less than $c$, with $c$ chosen so that the significance level is exactly $0.05$. What is the power of this test when $\theta = 0.8$? Give a fraction.