In order to encourage residents to save water, the water company stipulates that each household will be charged 2.4 yuan per ton for water consumption within 15 tons (including 15 tons) per month; if it exceeds 15 tons, the excess tonnage will be charged at 3 yuan per ton. Wenwenjia paid a total of 51 yuan last month. Do you know how many tons Wenwenjia used water last month?
为鼓励居民节约用水，自来水公司规定：每户每月用水15吨以内(含15吨)，按每吨2.4元收费；超过15吨的，其超出的吨数按每吨3元收费．文文家上个月共交51元，你知道文文家上个月用水多少吨吗？
<gadget id="calculator">15 * 2.4</gadget>
<output>36</output>

<gadget id="calculator">51 - 36</gadget>
<output>15</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 + 15</gadget>
<output>20</output>

<result>20</result>