Need help with a discrete math problem.

There are 30 students enrolled in a math class and each of them attended at least one of the first two lessons. The first lesson was attended by 27 students and the second lesson was attended by 20. How many students attended both of the first two lessons?

Question

Need help with a discrete math problem.

There are 30 students enrolled in a math class and each of them attended at least one of the first two lessons. The first lesson was attended by 27 students and the second lesson was attended by 20. How many students attended both of the first two lessons?

anonymous · Answer

This is a venn diagram problem. If you draw out a venn diagram with one side as students attending the 1st lecture, and one side attending the 2nd, and the middle being those who attended both you will be able to figure this out with guess and check.  Then you will see that 20 students attended both, and 7 attended only the first one.

anonymous · Answer

so if I did a venn diagram I would get 27 students in the first circle and 20 students in the second circle, since there are 30 students all together in both of the circles, the would intersect at 17? because 10+17 = 27, and 3 + 17 = 20

anonymous · Answer

so wouldn't it be 17 attended both?