Warehouse A has 72 tons of grain, and Warehouse B has 30 tons of grain. Some grains are transferred from Warehouse A to Warehouse B. The remaining grain in Warehouse A is twice that of Warehouse B. How many tons of grain were transferred from warehouse A to warehouse B?
甲仓库有72吨粮食，乙仓库有30吨粮食，从甲仓库调入一些粮食到乙仓库中，甲仓库剩下的粮食是乙仓库的2倍．从甲仓库调入了多少吨粮食到乙仓库？
<gadget id="calculator">72 + 30</gadget>
<output>102</output>

<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">102 / 3</gadget>
<output>34</output>

<gadget id="calculator">34 - 30</gadget>
<output>4</output>

<result>4</result>