First, let's make a proportion for the problem,
[tex] \frac{20t}{4g} = \frac{45t}{?g} [/tex]
T= teaspoon, g= gallon
[tex]20(x) = 45[/tex]
So 20 × what equals 45??
Simply divide both sides by 20,
[tex]x = \frac{9}{4} = 2.25[/tex]
The above number(s) is what we will × 20 and 4 by.
[tex] \frac{20t}{4g} \times \frac{2.25}{2.25} = \frac{45t}{9g} [/tex]
So for 45 teaspoons you'll need 9 gallons.