Question: Marta is planning a field trip to the zoo. Admission to the zoo is $12 per student and $20 per adult. What expression represents the total cost of admission for a group of $x$ students and $y$ adults?
1. **State the problem:** We need to find an expression for the total cost of admission for $x$ students and $y$ adults.
2. **Identify the costs:** Each student costs $12$ and each adult costs $20$.
3. **Write the expression:** The total cost is the sum of the cost for all students plus the cost for all adults.
4. **Formula:** Total cost $= 12 \times x + 20 \times y$
5. **Explanation:** Multiply the number of students $x$ by the student admission price $12$, and multiply the number of adults $y$ by the adult admission price $20$. Then add these two amounts to get the total cost.
6. **Final answer:** $$12x + 20y$$