At a lake, 40 people are water skiing or on the beach. Nine of the 12 people on the beach decide to swim. How many people are not on the beach?

3 not on the beach

To find out how many people are not on the beach, we need to subtract the number of people on the beach from the total number of people.

The total number of people at the lake is given as 40.

Out of the 40 people, 12 people are on the beach.

Now, out of the 12 people on the beach, 9 decide to swim.

To calculate the number of people not on the beach, we subtract the number of people on the beach (12) from the total number of people (40), and then subtract the number of people on the beach who decided to swim (9).

Total number of people not on the beach = Total number of people - Number of people on the beach - Number of people on the beach who decided to swim

Total number of people not on the beach = 40 - 12 - 9

Total number of people not on the beach = 19

Therefore, there are 19 people who are not on the beach.