The school originally had m tons of coal, but last month it burned 4 tons, and this month brought in another 20 tons. How many tons of coal does the school have now?
学校原有m吨煤，上个月烧了4吨，这个月又运来20吨，学校现在有煤多少吨．
<gadget id="calculator">(-4) + 20</gadget>
<output>16</output>

<result>16</result>