The ratio of blue to white paint is $2:5$, so there are $2+5=7$ total equal parts in the mixture. Blue paint makes up $\frac{2}{7}$ of the total mixture. Therefore, the number of cups of blue paint is $\frac{2}{7}\cdot 28$. So the correct expression is $\frac{2}{7}\cdot 28$.

Because the relationship is proportional, it has the form $y=kx$, where $k$ is the constant of proportionality. Use either given point to find $k$: $k=\frac{10}{4}=2.5$ or $k=\frac{30}{12}=2.5$. Then when $x=20$, substitute into $y=2.5x$: $y=2.5(20)=50$. So the correct answer is $50$.

The ratio of water to salt is $9:2$, so each full batch has $9+2=11$ total cups. The club has $33$ total cups, so the number of batches is $33÷11=3$. Each batch uses $2$ cups of salt, so the total amount of salt is $3\times 2=6$ cups. Therefore, the correct answer is $6$.

From the table, the car uses $4$ gallons for $120$ miles, so its rate is $\frac{4}{120}=\frac{1}{30}$ gallon per mile. For $300$ miles, the amount of gasoline used is $300\cdot \frac{1}{30}=10$ gallons. Therefore, the correct answer is $10$.

The ratio of small bags to large bags is $3:2$. That means the bags are sold in groups of $3+2=5$ total bags. Since 20 bags were sold, the number of groups is $20÷5=4$. Each group has 2 large bags, so the number of large bags sold is $4\times 2=8$. Therefore, the statement that must be true is that 8 large bags were sold.