The cost of sending mail is given as $0.51 for the first ounce and then every ounce after that costs $0.20.
For each letter, the cost would be expressed as;
[tex]\begin{gathered} C=0.51+0.20x \\ \text{Where x is the weight in ounces} \\ \text{For Stephanie's letter,} \\ 1.31=0.51+0.20x \\ \text{Subtract 0.51 from both sides} \\ 1.31-0.51=0.51-0.51+0.20x \\ 0.8=0.20x \\ \text{Divide both sides by 0.20} \\ \frac{0.8}{0.2}=x \\ 4=x \\ \text{Note that she paid \$0.51 for the first ounce} \\ \text{This means she paid for 1 ounce plus another 4 ounces} \\ \text{Stephanie's letter weighs 5 ounces} \end{gathered}[/tex]Therefore, Stephanie's letter weighs 5 ounces.
The correct answer is option D
