The third and fourth grades of Guangming Primary School have to water 780 trees. The third grade waters 60 trees every day for 7 days, and the rest is watered by the fourth grade in 4 days. How many trees are watered on average every day?
光明小学三年级和四年级要给780棵树浇水，三年级每天浇60棵，浇了7天，剩下的由四年级来浇，4天浇完，平均每天浇多少棵？
<gadget id="calculator">60 * 7</gadget>
<output>420</output>

<gadget id="calculator">780 - 420</gadget>
<output>360</output>

<gadget id="calculator">360 / 4</gadget>
<output>90</output>

<result>90</result>