The Wenchuan Earthquake Government urgently allocated a batch of materials. It took 4 hours to transfer in this batch of materials. The materials were transferred in 2 hours. There were 30 tons of materials in the warehouse. At this time, the materials were transferred out. After 2 hours, the warehouse had 10 tons of materials. and the speed of transferring out materials remain unchanged), how many hours will it take for this batch of materials to be transferred from the beginning to the time when all the materials are transferred out?
汶川地震政府紧急调拨一批物资，调进这批物资一共需要4小时，调进物资2小时，仓库有物资30吨，这时开始调出物资，2小时后仓库有物资10吨，(调进与调出物资的速度各自保持不变)，这批物资从开始调进到全部调出共需要多少小时？
<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">10 + 15</gadget>
<output>25</output>

<gadget id="calculator">10 / 25</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">2 + 2 + (2/5)</gadget>
<output>22/5 = around 4.4</output>

<result>22/5 = around 4.4</result>