I am going to show you a really cool trick.
The total attending is always 40. So if you start with no adults you have 40 children. As you increase the adult count the child count goes down proportionately. Thus by considering just one of them (I choose adult) the other is counted by inference. Thus we can plot the entrance cost by just observing the count of adult.
If all adult the cost would be:
If all children the cost would be:
The slope for part is the same as the slope for all. Thus we have