1. The problem is to find the value of $c$ given some context where the answer key states $c=20$. 2. Since the user only mentioned the answer key's value for $c$, we need to verify or solve for $c$ based on the original problem (not provided here). 3. Without the original equation or problem, we cannot directly solve for $c$. 4. If you provide the original equation or problem statement, I can help verify or solve for $c$. 5. Please share the full problem so I can assist you better.