There are two warehouses, A and B. The grain stored in warehouse A is three times that of warehouse B. If warehouse A transfers 28 tons to warehouse B, warehouse A will be 4 tons more than warehouse B. How many tons of grain are stored in warehouse B? Use equations and formulas in two ways.
有甲、乙两个仓库，甲仓库存粮是乙仓库的3倍，如果甲仓库调28吨到乙仓库，则甲仓库还比乙仓库多4吨，原来乙仓库存粮多少吨？用方程和算式两种方法．
<gadget id="calculator">28 + 28 + 4</gadget>
<output>60</output>

<gadget id="calculator">60 / 2</gadget>
<output>30</output>

<result>30</result>