Analysis: You can do this problem with intersecting figures. Dance class and calligraphy class count as one. The union number of the two classes is 58, but the total number is 48, so we can know that the number of the intersection part of the two classes (that is, learning both dance and calligraphy) is 10.
The answer to the second question: 13
This problem can be solved by equation substitution. Let other flags have x faces, and then we can list the equation: x+ yellow flag =24 x+ red flag =20 red flag+yellow flag = 18. Add the first equation and the second equation until the new equation 2x+ red flag+yellow flag =44, because red flag and yellow flag are.
The answer to the third question: 20 people.
The idea of the third question is the same as that of the first question, but don't forget to add two people who didn't answer the two questions correctly. That is 15 (for the first question)+10 (for the second question) -7 (for both repeated additions and subtractions) +2 (neither is right).