学校举办排球赛，某班50名同学中共有15名同学参赛，后来又举办了田径赛这个班有20名同学参赛，已知两项比赛都参加有7名同学．这个班共有______名同学两项比赛都没有参加．
设参加排球赛的同学为集合A，参加田径赛的同学为集合B
则集合A的元素个数card（A）=15，集合B的元素个数为card（B）=20，两项比赛都参加的人数为card（A∩B）=7
∴两项比赛都不参见的人数为：50-card（A）-card（B）+card（A∩B）=50-15-20+7=22
故答案为：22
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| Football is, I think, the most popular sport in the world today. One can go to any of the important 1 , shouting for one side or the 2 . One of the most 3 things about football is that 4 a small boy can tell you the names of the 5 in most of the important teams. He has pictures of them and knows the result of lots of matches. 6 many years ago, each country had different rules for 7 own football games. In 1863, a group of people met in England to 8 this. These rules are the 9 today's. Now teams from all over the world take part in the famous World Cup. It is held every four years to see which is 10 . | ||||
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