#### Answer

$C=300$

#### Work Step by Step

To find out the solution we need to follow the four-step procedure:
Step (1). As $C$ varies jointly with $A$ and $T$, we have
$C=k\times A\times T$
Here $k$ is a constant.
Step (2).
Substitute $A=2100$ , $T=4$ and $C=175$ in $C=k\times A\times T$ to get the value of k.
$\begin{align}
& 175=K\times 2100\times 4 \\
& K=\frac{175}{2100\times 4} \\
& K=\frac{1}{48} \\
\end{align}$
Step (3). Substitute the value of $k$ into the main equation.
$C=\left( \frac{1}{48} \right)\cdot A\cdot T$
Step (4). Substitute $A=2400\,\text{ and }\,T=6$ to get:
$\begin{align}
& C=\frac{1}{48}\times 2400\times 6 \\
& C=300 \\
\end{align}$