How many milliliters of a 15% saline solution and how many milliliters of a 40% saline solution must be mixed to produce 10 milliliters of a 30% saline solution?
volume of 15% solution=
volume of 40% solution =

X - volume of 15% saline solution. Theres is 15% x = 0,15 x saline solution
y - volume of 40% saline solution. There is 40% y = 0,4y saline solution

You want to produce 10 mililiers  so x+y= 10 

And this saline has to be 30%, so you've got  30 % * 10 = 0,3 * 10 = 3 ml saline. So:
0,15x + 0,4y = 3
And you've got system of equations:


So
Volume of 15% saline solution : 4 mililiters
Volume of 40% saline solution: 6 mililiters. 

Do the inverse operation