Noah knows that his friend Ryan uses 2 small chlorine tablets a week to maintain his 10,000 gallon pool
Direct variation is expressed with the formula y=kx. Where k is the constant of variation, x is the independent variable and y is the dependent variable.
In your question, x will equal gallons of water and y is equal to chlorine tablets. The amount of water in a pool does not depend on how many chlorine tablets there are that's why gallons of water is independent and chlorine tablets will be dependent (on how many gallons there are).
Plug in what you know now to the direct variation equation.
(2) = k(10,000) solve for k
(2)/(10,000) = k
1/5,000 = k (or in decimal form for calculators it would be 0.0002)
For the generic formula for this problem, substitute the variables the problems gives you (C and G). X stood for gallons and Y stood for chlorine tablets. And for any G value the equation would look like
C=0.0002G or C=(1/5000)G