1) I would agree: add all the different categories of expenses to get an expression for the total expense.
2) Graph the "revenue" formula they gave you and the 'expenses" formula you just came up with.
3) Eye-ball the graph (or use the "intersection" utility on your graphing calculator) to approximate the x-value (okay, t-value) for which the two lines cross.
4) Eye-ball the graph, and locate the x-value for which the two lines are the farthest apart.
5) Divide the expression for "C" by the expression for "E" to get an expression for the percentage that C is of E. Graph the result. Eye-ball to find the x-value for which the graph is lowest. The y-value for that point will be the percentage (as a decimal, of course; you'll need to convert to a percentage).