A customer has to decide how many of products $x$ and $y$ to buy. The price of $x$ is $2x^{1/2}$ and the price of $y$ is $4y^{1/2}$. The cost of $x$ is 10 dollars and the cost of $y$ is 5 dollars. The customer wishes to spend a total of 90 dollars for both $x$ and $y$. How many units of $y$ should he buy?

I started with $10x + 5y = 90$ with $y=18 - 2x$ and differentiating got $\frac{1}{x^{1/2}} /4(18-2x)^{1/2} =10/5$, but am lost after that.