a, b, c are three numbers arranged in descending order, and ab=bc. The difference between the product of the first two numbers and the product of the last two numbers is 280. If b=35, what is c?
a、b、c是从大到小排列的三个数，并且a-b=b-c．前两个数的积与后两个数的积之差是280，如果b=35，那么c是多少．
<gadget id="calculator">70 - 8</gadget>
<output>62</output>

<gadget id="calculator">62 / 2</gadget>
<output>31</output>

<result>31</result>