The postman picks up 6 letters a day. The first time is at 7:00 am and the last time is at 5:00 pm. If the time intervals for obtaining the letter are the same, how long is the fourth time for obtaining the letter?
邮递员每天要取6次信．第一次是早晨7时，最后一次是下午5时．如果取信的时间间隔相同，那么第四次取信是多少时．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">10 / 5</gadget>
<output>2</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<gadget id="calculator">17 - 4</gadget>
<output>13</output>

<result>13</result>