Darwin starts by mixing 4 cups of crystals with 9 cups of water. How many more cups of water should he add to have a 2:5 ratio?

Answer 1

4/(9+x) = 2/5

18+2x = 20

2x = 20-18=2

x = 2/2 = 1 cup of water more will change the ratio to 2:5

4/(9+1) = 4/10 = 2/5