A pile of logs stacked in a trapezoidal shape will decrease by 2 each time it goes up. It is known that there are 10 logs in the top layer and 20 logs in the bottom layer. How many logs are there in total in this pile.
一堆梯形堆放的原木，每往上一层都会减少2根，已知最上层有10根，最下层有20根，这堆原木共有多少根．
<gadget id="calculator">10 + 20</gadget>
<output>30</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 + 1</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<result>90</result>