So we're given that we bought 12 sandwiches. Focus on the sandwiches.
You have 20 salads and 15 sandwiches in a bag, you withdraw 12 sandwiches. What are you left with?
Answer: 20 salads and 3 sandwiches.
The question you ask yourself now is how many salads does 3 sandwiches equal?
Well I was going to say(assuming equality 20 salads = 15 sandwiches), 20/15*3...
but since we were not told equality...this question is probably incomplete, or more info is needed.
Seeing it's a teaching course, just being able to think logically and explan yourself will suffice most times in these types of problems.