The postman picks up the letters in the mailbox four times a day. The first time is at 7:00 am and the last time is at 7:00 pm. If the time interval of getting the letter is the same, what is the moment of the third time getting the letter.
邮递员每天4次取信箱里的信．第一次是早晨7时，最后一次是下午7时．如果取信的时间间隔相同，那么第3次取信的时刻是多少．
<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">7 + 8</gadget>
<output>15</output>

<result>15</result>