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?
On the second day, Arvin ran 2 + 1 = 
<gadget id="calculator">2+1</gadget>
<output>3</output>
3 kilometers.
On the third day, he ran 3 + 1 = 
<gadget id="calculator">3+1</gadget>
<output>4</output>
4 kilometers.
On the fourth day, he ran 4 + 1 = 
<gadget id="calculator">4+1</gadget>
<output>5</output>
5 kilometers.
So on the fifth day, he ran 5 + 1 = 
<gadget id="calculator">5+1</gadget>
<output>6</output>
6 kilometers.

<result>6</result>