The solution to exercise 216 from the 4th-grade mathematics textbook by Arginskaya et al. requires dividing the 100 km total distance into 4 parts based on a 1:3 ratio, finding that 25 km was traveled when the camp was made. This calculation is verified by confirming the remaining distance is three times the traveled distance. For more detailed teaching guides and full textbook solutions, you can consult the official Methodological Recommendations provided by the publisher.