A store sells a commodity, 5 pieces are purchased at 120 yuan, and 4 pieces are sold at 180 yuan, then how many pieces of the commodity must be sold in order to make a profit of 4200 yuan.
商店出售一种商品，进货时120元5件，卖出时180元4件，那么商店要盈利4200元必须卖出多少件该商品．
<gadget id="calculator">180 / 4</gadget>
<output>45</output>

<gadget id="calculator">120 / 5</gadget>
<output>24</output>

<gadget id="calculator">45 - 24</gadget>
<output>21</output>

<gadget id="calculator">4_200 / 21</gadget>
<output>200</output>

<result>200</result>