Yucai Primary School has 140 students in the upper grade, 175 students in the middle grade, and 210 students in the lower grade. The number of students in each class of these three grades is 35. How many classes are there in these three grades?
育才小学高年级有学生140人，中年级有学生175人，低年级有学生210人，这三个年级每个班的人数都是35人，求这三个年级一共有多少个班？
<gadget id="calculator">140 + 175 + 210</gadget>
<output>525</output>

<gadget id="calculator">525 / 35</gadget>
<output>15</output>

<result>15</result>