Answer
$y=240$
Work Step by Step
To find out the solution we need to follow the four-step procedure:
Step (1). As $y$ varies jointly as $x$ and $z$, we have
$y=kxz$
Here $k$ is a constant.
Step (2).
Substitute $x=2$ , $z=5$ and $y=25$ in $y=kxz$ to find the value of k
$\begin{align}
& 25=k\times 2\times 5 \\
& k=\frac{25}{10} \\
& k=\frac{5}{2} \\
\end{align}$
Step (3). Substitute the value of $k$ into the main equation.
$y=\left( \frac{5}{2} \right)xz$
Step (4). Substitute the values $x=8\,\text{ and }\,y=12$ to get:
$\begin{align}
& y=\frac{5}{2}\times 8\times 12 \\
& y=5\times 4\times 12 \\
& y=240 \\
\end{align}$
