In a survey using a stratified design, let $N_h$ be the stratum size, $\sigma_h$ the stratum standard deviation, and $c_h$ the cost per unit in stratum $h$. If the total cost is $C = c_0 + \sum_{h=1}^H c_h n_h$, what is the optimal sample size $n_h$ that minimizes the variance of the mean estimator for a fixed budget $C$?