#### Answer

Required amount of water : 9 gallons

#### Work Step by Step

( Note x:y is same as $\frac{x}{y}$)
Given:
8 teaspoons of weed killer are to mixed with 2 gallons of water
Thus ratio of weed killer to water = 8:2 = 4:1
If there are 36 teaspoons of weed killer, and 'x' gallons of water,
Then the ratio should be 4:1
Thus $\frac{36}{x}$ = $\frac{4}{1}$
Thus x = $\frac{36}{4}$ = 9
So 9 gallons of water are needed.