Answer
240
Work Step by Step
Solving Variation Problems (see p. 424)
1. $\ \ $Write an equation that models the given English statement.
2. $\ \ $Substitute the given pair of values into the equation in step 1 and find the value of k, the constant of variation.
3. $\ \ $Substitute the value of k into the equation in step 1.
4. $\ \ $Use the equation from step 3 to answer the problem's question.
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1.
$y$ varies directly as $x$ or $y$ is directly proportional to $x$.
$y=kx$
$y$ varies directly as $z$ or $y$ is directly proportional to $z$.
$y=kz$
Combined: $y=kxz$
2.
$25=k(2)(5) $
$25=10k \qquad /\div 10$
$k=\displaystyle \frac{25}{10}=2.5$
3.
$y=2.5xz$
4.
$y=2.5(8)(12) =240$
