50 ounces of 15% saline solution contains 0.15*50 = 7.5 ounces of salt.
Let x be the amount of pure water you add to this solution. Then you end up with (x + 50) ounces of this new solution. The amount of salt stays the same.
You want the new solution to have a concentration of 10%. This means you need
7.5 / (x + 50) = 0.1
==> 7.5 = 0.1(x + 50)
==> 7.5 = 0.1x + 5
==> 2.5 = 0.1x
==> 2.5/0.1 = 0.1x/0.1
==> x = 25
You are given two equations, solve for one variable in one of the equations. Say you solved for x in the second equation. Then, plug in that value of x in the x of the first equation. Solve this (first) equation for y (as it should become apparent) and you'll get a number value. Plug in this numerical value of y into the y of the second equation. Solve for x in the second equation. And there you have it: (x, y)
Buying something that costs more than $1.00 can be an example.
