First find the total number of notebooks sold: $12+18+15+21=66$. Since the ratio of spiral notebooks to total notebooks is $2:3$, the number of spiral notebooks is $\frac{2}{3}\cdot 66=44$. Therefore, the correct answer is $44$.

Because the garage charges at a constant rate, the cost is proportional to the number of hours parked. If 2 hours costs $6,thentheunitrateis$\frac{6}{2}=3$dollarsperhour.Therefore,parkingfor1hourmustcost$3$.$

The scale is $1$ inch for every $3$ feet, so divide each actual dimension by $3$ to get the blueprint dimensions. The room's length on the blueprint is $12÷3=4$ inches, and its width is $9÷3=3$ inches. The area on the blueprint is $4\times 3=12$ square inches. Therefore, the correct answer is $12$.

First find the unit rate: $\frac{90\text{ miles}}{2\text{ hours}}=45$ miles per hour. Then multiply by 5 hours: $45\times 5=225$. So the shuttle bus will travel $225$ miles.

A ratio of $1:7$ means there are $1+7=8$ total equal parts in the mixture. The fertilizer is $1$ of those $8$ parts, so the amount of fertilizer is $\frac{1}{8}\cdot 40=5$ cups. Therefore, the correct answer is $5$.