There are 50 students in class 6 and 3, 20 of them can play badminton, 8 can play both badminton and table tennis, 9 can’t play badminton and table tennis, how many can play table tennis ．
六3班有学生50人，会打羽毛球的有20人，羽毛球和乒乓球都会打的有8人，不会打羽毛球又不会打乒乓球的有9人，会打乒乓球的有多少人．
<gadget id="calculator">50 - 20 - 9</gadget>
<output>21</output>

<gadget id="calculator">8 + 21</gadget>
<output>29</output>

<result>29</result>