A batch of concrete is made by mixing cement, yellow sand and gravel. Among them, 8 tons of cement were used, and 7 tons more yellow sand was used than cement, and the number of tons of stones was exactly twice the sum of the tons of cement and yellow sand. How many tons of stones are used?
一批混凝土是由水泥、黄沙、石子混合拌成的．其中，水泥用了8吨，黄沙比水泥多用了7吨，石子的吨数正好是水泥与黄沙吨数和的2倍．石子用了多少吨？
<gadget id="calculator">8 + 7 + 8</gadget>
<output>23</output>

<gadget id="calculator">23 * 2</gadget>
<output>46</output>

<result>46</result>