Hongfu Supermarket buys a batch of table salt, sells 40% of this batch of salt in the first month, and sells 420 bags of this batch of salt in the second month. At this time, the ratio of sold to remaining table salt is 3 : 1, how many bags of this batch of salt are purchased by Hongfu Supermarket?
宏富超市购进一批食盐，第一个月售出这批盐的40%，第二个月又售出这批盐的420袋，这时已售出的和剩下食盐的数量比是3：1，则宏富超市购进的这批食盐有多少袋．
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (2/5) - (1/4)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">420 / (7/20)</gadget>
<output>1_200</output>

<result>1_200</result>