In preparation for his mountain climbing, Arvin wants to run a total of 20 kilometers in a week. On the first day, he ran 2 kilometers. On each subsequent day, he increased his running distance by 1 kilometer over the previous day. If he runs for 5 days a week, how many kilometers did he run on the 5th day? Let's answer step by step:
On the second day, Arvin ran 2 + 1 = 3 kilometers. On the third day, he ran 3 + 1 = 4 kilometers. On the fourth day, he ran 4 + 1 = 5 kilometers. So on the fifth day, he ran 5 + 1 = 6 kilometers. The answer: 6.