Math Help - Write a solution for this word problem?

30 liters of a 40% acid solution is obtained by mixing a 25% solution with a 50% solution.
Write a system of equations in which one equation represents the amount of final mixture required and the other represents the percent of acid in the final mixture. Let x and y represent the amounts of the 25% and 50% solutions, respectively.

I know how to solve these type of equations, but I'm not sure how to make equations to fit this!

Your first equation: The sum of the two solutions has volume of 30 liters.

Your second equation: The sum of 25% of the first solution and 50% of the second solution is equal to 40% of the entire solution of 30 liters