Final answer:
The 7th grader's mileage per gallon of milk is 34.2 miles per gallon of milk.
Step-by-step explanation:
To find the 7th grader's mileage per gallon of milk, we divide the distance walked (205 miles) by the amount of milk energy used (6 gallons). This gives us a ratio of 34.17 miles per gallon of milk. Rounding to the nearest tenth, the 7th grader's mileage per gallon of milk is 34.2 miles per gallon of milk.