Answer:
1.
![C=21x.](https://img.qammunity.org/2020/formulas/mathematics/high-school/wa4nmficlq8rwopwuk91mci55nsdj71t6s.png)
2.
![C=(8x+104).](https://img.qammunity.org/2020/formulas/mathematics/high-school/ccvyzqrnhlms2i5evkel0qo2mqv3lentew.png)
Explanation:
Let the number of people in the group be x. Denote the total cost as C.
1. If the regular cost is $21 per person, then the total regular cost for x people is
![C=21x.](https://img.qammunity.org/2020/formulas/mathematics/high-school/wa4nmficlq8rwopwuk91mci55nsdj71t6s.png)
2. If the special cost is $8 per person, then the total special cost for x people is
There is an additional $104 fee for a private room, thus the total special cost is
![C=(8x+104).](https://img.qammunity.org/2020/formulas/mathematics/high-school/ccvyzqrnhlms2i5evkel0qo2mqv3lentew.png)