# Suppose y varies jointly as x and z. How do you find y when x=6 and z=8, if y=6 when x is 4 and z is 2?

Aug 3, 2017

$y = 36$

#### Explanation:

If $y$ varies jointly as $x$ and $z$, we can write this as

$y = k x z$

where $k$ is the constant of proportionality (we'll be finding this)

In the situation, we're given that $y = 6$ when $x = 4$ and $z = 2$, so let's plug those in:

$6 = k \left(4\right) \left(2\right)$

$k = \frac{3}{4}$

Now that we know the proportionality constant (which stays the same), we can use it in solving for $y$ when $x$ is $6$ and $z = 8$:

y = (3/4)(6)(8) = color(blue)(ulbar(|stackrel(" ")(" "36" ")|)