Final answer:
Ellen can make three portions of trail mix by finding the greatest common divisor of the number of nut and dried fruit bags, which is 3.
Step-by-step explanation:
To determine the greatest number of portions Ellen can make, we need to find the greatest common divisor (GCD) of the numbers of bags of nuts and dried fruit. For 9 bags of nuts and 6 bags of dried fruit, the GCD is 3. Therefore, Ellen can make three portions of trail mix, each containing an identical combination of 3 bags of nuts and 2 bags of dried fruit, with no bags leftover.