A pile of coal is planned to burn 0.5 tons per day, and it can be burned for 40 days. If it burns 0.4 tons per day, how many days can it be burned? (Solve with ratio)
一堆煤，计划每天烧0.5吨，可以烧40天，如果每天烧0.4吨，可以烧多少天？(用比例解)
<gadget id="calculator">0.5 * 40</gadget>
<output>20</output>

<gadget id="calculator">20 / 0.4</gadget>
<output>50</output>

<result>50</result>