First find each van's average speed from the table using $\text{speed}=\frac{\text{distance}}{\text{time}}$.

For van $A$:

So van $A$ traveled at $36$ mph.

For van $B$:

So van $B$ traveled at $42$ mph.

For van $C$:

So van $C$ traveled at $48$ mph.

Now match these to the model:
- van $A$: $r-6=36$
- van $B$: $r=42$
- van $C$: $r+4=48$

Each equation gives the same value:

So the correct answer is $42$.

Let $x$ be the number of liters of solution X and $y$ be the number of liters of solution Y. Since the total mixture is 30 liters, $x+y=30$. The amount of salt in the final mixture is $0.32(30)=9.6$ liters. The salt equation is $0.20x+0.50y=9.6$. Substitute $x=30-y$ into the salt equation: $0.20(30-y)+0.50y=9.6$. This gives $6-0.20y+0.50y=9.6$, so $6+0.30y=9.6$. Then $0.30y=3.6$, so $y=12$. Therefore, $x=18$. Since $18>12$, the volume of solution X was greater than the volume of solution Y. So choice A must be true.

The original ratio of flour to sugar is $3:2$. To keep the same ratio when using 15 cups of flour, multiply both parts of the ratio by 5, since $3\times 5=15$. That gives $2\times 5=10$ cups of sugar. But the baker uses 12 cups of sugar, which is 2 cups more than needed to keep the ratio the same. Therefore, the mixture has a greater proportion of sugar than the original recipe, so it must be too sweet compared with the original. Choice A is correct.

Because the tank is cylindrical, its cross-sectional area is constant, so the volume of water is proportional to the height of the water level. If 2 feet of water corresponds to 180 gallons, then 1 foot corresponds to $\frac{180}{2}=90$ gallons. A water level of 5 feet therefore corresponds to $5\times 90=450$ gallons. So the correct answer is $450$.

The ratio of concentrate to water is $2:5$. That means for every $2$ cups of concentrate, there are $5$ cups of water. To find the amount of water for $c$ cups of concentrate, multiply $c$ by the ratio $\frac{5}{2}$:

So the correct expression is $\frac{5}{2}c$.