Question 946185
Let a be the fixed cost and b be the cost per person. Suppose you know $1000 for 30 people and $2000 for 80 people. Set up a system of equations in terms of a and b. 
----------------
a + b*30 = 1000
a + b*80 = 2000
----------------
Subtract and solve for "b"::
50b = 1000
b = 20 (cost per person)

-----------
Solve for "a"::
a + 20*30 = 1000
a + 600 = 1000
a = $400 (fixed cost)
-----
Cheers,
Stan H.
---------------
==============================