Let's use the variable "J" to represent the number of posters Jess did, and "G" to represent the number of posters Gabby did.
If they made 20 posters together, we have:
![J+G=20](https://img.qammunity.org/2023/formulas/mathematics/college/69j4ozxri05fi6p4ohjnmq8yq88zvq28c5.png)
Then, if Jess made 5 posters (that is, J = 5), we can calculate how many posters Gabby did:
![\begin{gathered} 5+G=20 \\ G=20-5 \\ G=15 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/aybso4j6gfyqapzn016pyzcfu69s6c0zqt.png)
So Gabby did 15 posters.