A store buys and sells a batch of dry cleaning machines, the purchase price is 400 yuan each, and the selling price is 420 yuan each. What percentage of profit can be made per unit?
商店买卖一批干洗机，进货价每台400元，售出价每台420元．每台可获利百分之几？
<gadget id="calculator">420 - 400</gadget>
<output>20</output>

<gadget id="calculator">20 / 400</gadget>
<output>1/20 = around 0.05</output>

<result>1/20 = around 0.05</result>