Need to know how to do this problem

Question

A hair dye is made by blending a 7% hydrogen peroxide solution and a 4% hydrogen peroxide solution. How many millilitres of each solution are used to make a 300-millilitre solution that is 5% hydrogen peroxide?

use substitution or elimination method.

include a let statement, equation and solution.

Answer 1

x+y= 300

x=300-y

7x+4y = 5(300) =1500

7(300-y) + 4y= 1500

2100-1500 = 3y

y = 200 ml of 4%

x= 100 ml of 7%