A tech startup projects quarterly revenue using the model $R(q) = 50q - 2q^2 + 0.1q^3$ thousand dollars, where $q$ is the quarter number. Operating costs are $C(q) = 10 + 3q + 0.05q^2$ thousand dollars. Profit is $P(q) = R(q) - C(q)$. After how many quarters is the marginal profit (rate of change of profit) equal to zero?