I'm completely lost with this math problem: While Greg was camping with his family for a week, it rained for 3 days. When he looked at the weather records, he saw that the amount of rain was 3 mm, 6 mm, and 5 mm on the three days. During the same week, it rained 26 mm at his house. How much less rain did Greg experience while camping?
Can you give me a hand?