The 9th graders are selling tickets to raise
money for a class field trip. They are selling
student tickets for $5 and adult tickets for $9.
The 9th graders received a total of $240 from
sales. In total, 40 tickets were sold.

There were 30 children and 10 adults.

Step-by-step explanation:

Since they sold tickets to adults and childrens, the sum of those tickets must be equal to the total tickets that were sold, so we can mount our first equation as shown below:

adult + children = 40

Each adult ticket costs $9 and each children costs $5 and the total recieved was $240, therefore:

9*adult + 5*children = 240

We can mount the following equation system:

adult + children = 40

9*adult + 5*children = 240

We will multiply the first equation by -5 and sum both equations:

-5*adult -5*children = -200

9*adult + 5*children = 240

4*adult = 40

adult = 40 / 4

adult = 10

adult + children = 40

children = 40 - adult = 40 - 10 = 30

There were 30 children and 10 adults.

6 equilateral triangles