If you divide the top number, then you have to divide the bottom number with the same number, and when you can't do that anymore, they have to be divided by 1, and they will stay the same.
F = 15% v = 5% 0.15f + 0.05v = 0.14*10 15f + 5v = 140 3f + v = 28 f + v = 10 (total volume) ---------- Subtract 2f = 18 f = 9 liters of 15% v = 1 liter of 5%
