A tour group buys tickets, 15 yuan for children, 30 yuan for adults, a total of 56 people, and 1230 yuan to buy tickets. How many children are there in this tour group?
一个旅行团买票，儿童15元，成人30元，共56人，买票用去1230元，这个旅游团共有多少名儿童？
<gadget id="calculator">1_230 - 840</gadget>
<output>390</output>

<gadget id="calculator">30 - 15</gadget>
<output>15</output>

<gadget id="calculator">390 / 15</gadget>
<output>26</output>

<gadget id="calculator">56 - 26</gadget>
<output>30</output>

<result>30</result>