The postman delivered 1,800 letters in June. This month he sent letters for 4 hours a day, how many letters per hour on average?
邮递员六月共送信1800封．这个月他每天送信4小时，平均每小时送信多少封？
<gadget id="calculator">4 * 30</gadget>
<output>120</output>

<gadget id="calculator">1_800 / 120</gadget>
<output>15</output>

<result>15</result>