"Individual Tax Law" stipulates that the part of personal monthly salary income exceeding 3,500 yuan should pay personal income tax. Wang Gang's monthly salary is 4,200 yuan, and the excess part is subject to personal income tax at 3%. How much tax should he pay?
《个税法》规定，个人月工资收入超过3500元的部分，应缴纳人个所得税．王刚每月工资4200元，超过的部分按3%缴纳个人所得税．他应缴纳多少元的税．
<gadget id="calculator">4_200 - 3_500</gadget>
<output>700</output>

<gadget id="calculator">3 / 100</gadget>
<output>3/100 = around 0.03</output>

<gadget id="calculator">700 * (3/100)</gadget>
<output>21</output>

<result>21</result>