In the sixth grade of Hanjiang Primary School, (1/2) students participated in the Chinese reading competition, (9/20) students participated in the "Into the Kingdom of Mathematics" TV invitational competition, and 20 students participated in both competitions, (7/20) 40) students did not participate in these two competitions. How many students in the school's sixth grade participated in the "Entering the Kingdom of Mathematics" TV invitational contest?
汉江小学六年级有(1/2)的同学参加了语文读书竞赛，(9/20)的同学参加了"走进数学王国"电视邀请赛，两项竞赛都参加的同学有20人，(7/40)的同学这两项竞赛都没有参加．该校六年级参加"走进数学王国"电视邀请赛的同学有多少人．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">9 / 20</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">7 / 40</gadget>
<output>7/40 = around 0.175</output>

<gadget id="calculator">(7/40) - 1</gadget>
<output>-33/40 = around -0.825</output>

<gadget id="calculator">(1/2) + (9/20) + (-33/40)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">20 / (1/8)</gadget>
<output>160</output>

<gadget id="calculator">160 * (9/20)</gadget>
<output>72</output>

<result>72</result>