This is simply the formula for mass percentage. Next we do a little algebra. We bring the 100 over. So we divide five by 100, get 0.5,
then we bring the bottom over, multiply this whole bottom by 0.5, so we get 0.05 times 180 plus x. When we multiply this out, we get nine plus 0.5 x. We equate that to x because the x was left on the other side.