If x/y = 3/z, then 9z^2 =  Choose one
A) y²/ x²  B) x²y²  C) 81y²/ x².  D) 9y/x  E) x²/y²
This question is most easily solved by isolating y in the equation and substituting into the expression 9z²:   x/y = 3/z   x = 3y/z   3y = xz   z = 3y/x.   If we substitute 3y/xinto the expression for z, we get:   9(3y/x)² = 9(9y²/ x²) = 81y²/ x².   The correct answer is choice (81y²/ x².)
<result>C</result>
If x/y = 3/z, then 9z^2 =