Chemical solution question.

A chemical manufacturer has an order for 500 litres of a 25% acid solution (i.e. 25% by volume is acid). Solutions of 30% and 18% are available in stock.

How much acid is required to produce 500 litres of 25% acid solution?

The manufacturer wishes to make up the 500 litres from a mixture of 30% and 18% solutions.

Let x denote the amount of 30% solution required.

Let y denote the amount of 18% solution required.

Use simultaneous equations in x and y to determine the amount of each solution.

I'm pretty sure once I find what the simultaneous equations are, I will be able to solve them.

Thanks. :D