Question 99074
The cost for an extra 15 guests is $60 less than for 25 people.

The cost per person for those guests must therefore be -$4 per person.

The amount charged for the 40 guests would be -$160 so the fixed cost would be $400.

Check 25 people @ -$4 =-$100  Add on a fixed cost of $400 gives a total cost of $300.

Check 40 people @ -$4 =-$160  Add on a fixed cost of $400 gives a total cost of $240.

This of course does not make sense for a greater number of people to result in a smaller cost so the original problem posted on this site seems to have an error in it.