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
[tex]x+x+3+x+x+8=31[/tex]Collecting the like terms, we get
[tex]\begin{gathered} x+x+x+x+11=31 \\ 4x=31-11 \\ 4x=20 \\ \text{Dividing both sides by 4, we get} \\ x=\frac{20}{4}=5 \end{gathered}[/tex]So, the number of bills they received is 5.
Hence, the correct answer is 5 bills.