Elena is buying food for a school picnic and has $40 to spend on sandwiches. She can buy 5 turkey sandwiches and 5 avocado sandwiches for $40, or she can buy 6 turkey sandwiches and 3 avocado sandwiches for $39. Based on the prices of these two options, what is the cost of 1 turkey sandwich and 2 avocado sandwiches? Explain.