We know that you need 18 teaspoons of weed killer and 3 gallons of water. We can represent this as a fraction as 18/3. We can simplify this fraction. 18/3 simplified is 6/1. Next, we can multiply the numerator and denominator by 4. (6*4 is 24.) This gives us 24/4. This means that for 24 teaspoons of weed killer, we need 4 gallons of water.