factory carried out energy saving week activities. It saved an average of 3.6 tons of coal per day in the first 3 days, and saved a total of 17.2 tons of coal in the next 4 days. How many tons of coal was saved on average this week?
某工厂开展节约能源周活动，前3天平均每天节约用煤3.6吨，后4天共节约用煤17.2吨，这一周平均每天节约用煤多少吨．
<gadget id="calculator">3.6 * 3</gadget>
<output>10.8</output>

<gadget id="calculator">10.8 + 17.2</gadget>
<output>28</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">28 / 7</gadget>
<output>4</output>

<result>4</result>