The Solution.
Let the number of ads be = x
Number of Letters = x+3
Number of bills = x
Number of magazines = x+3+5 = x+8
Given that the total number of mail is 31, we have
![x+x+3+x+x+8=31](https://img.qammunity.org/2023/formulas/mathematics/college/b0d88sqerx7a7aazti70gfklakp0qqdjl9.png)
Collecting the like terms, we get
![\begin{gathered} x+x+x+x+11=31 \\ 4x=31-11 \\ 4x=20 \\ \text{Dividing both sides by 4, we get} \\ x=(20)/(4)=5 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/dqy384437jqmq3y62pmoxcnjxar4rbtynp.png)
So, the number of bills they received is 5.
Hence, the correct answer is 5 bills.