family can save 4 tons of water a month, and how many tons of water can 30 families save in a year.
一个家庭一个月可节水4吨，30个家庭全年可节水多少吨．
<gadget id="calculator">4 * 12 * 30</gadget>
<output>1_440</output>

<result>1_440</result>