In an arithmetic sequence, the sum of the 1st, 5th, and 9th items is 117, and the sum of the 3rd, 7th, and 11th items is 141. What is the 30th item of this arithmetic sequence?
一个等差数列，第1项、第5项、第9项的和是117，第3项、第7项、第11项的和是141，那么这个等差数列的第30项是多少．
<gadget id="calculator">117 / 3</gadget>
<output>39</output>

<gadget id="calculator">141 - 117</gadget>
<output>24</output>

<gadget id="calculator">24 / 3</gadget>
<output>8</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">39 - 16</gadget>
<output>23</output>

<gadget id="calculator">30 - 1</gadget>
<output>29</output>

<gadget id="calculator">4 * 29</gadget>
<output>116</output>

<gadget id="calculator">23 + 116</gadget>
<output>139</output>

<result>139</result>