% solutions problem

[doofus]

Here is the problem,a company has an order for 500 gallons of a 35% acid solution,they only have a 25% and a 50% solution on hand.How many gallons of each type of solution should be mixed together to fill the order.I'm guessing here and saying 450 gallons of the 25% and 50 gallons of the 50%, but I would like to know how to work this out.Thank you,this is for my child.

 

[galactus]

Let x=gallons of 25% solution
y=gallons of 50% solution

.25x+.50y=500(.35)
x+y=500

Solve the system.