Teacher Li goes to Meishan to study and goes there once a day. The one-way fare is 13 yuan. If the monthly ticket costs 390 yuan, please help Teacher Li to calculate.
李老师到眉山学习，每天往返一次，单程车费是13元，如果买月票要390元，请你帮李老师算一算，李老师至少要到眉山学习多少天买月票才划算？
<gadget id="calculator">13 * 2</gadget>
<output>26</output>

<gadget id="calculator">390 / 26</gadget>
<output>15</output>

<gadget id="calculator">15 + 1</gadget>
<output>16</output>

<result>16</result>