Each set was priced at 70 yuan, 30 yuan, and 20 yuan. I bought a total of 47 sets of three books and paid 2120 yuan. Among them, the purchase quantity of 30 yuan per set is twice that of 20 yuan per set, so how many sets of books are priced at 70 yuan.
每套定价分别是70元、30元、20元的三种书共买了47套，交了2120元钱．其中每套30元的购买数量是每套20元的购买数量的2倍，那么定价是70元的书有多少套．
<gadget id="calculator">3_290 - 2_120</gadget>
<output>1_170</output>

<gadget id="calculator">210 - 20 - 60</gadget>
<output>130</output>

<gadget id="calculator">1_170 / 130</gadget>
<output>9</output>

<gadget id="calculator">2 * 9</gadget>
<output>18</output>

<gadget id="calculator">47 - 9 - 18</gadget>
<output>20</output>

<result>20</result>