Lawnmower accounting system in C

I am required to do a lawnmower accounting system in C, I have paid out the following ideas what to include in the source code.

1) Main function, this will provide the menu interface to get to all the rest of them, it will basically be a switch statement that calls the other functions and returns their values.

2) Square foot to acreage and acreage to square foot conversion, this function should take a numeric value and an alphabetic value and convert from one to the other.  use calculations in square footage for final pricing.

3) Mowing price calculation - based on the acreage come up with a price.  Lets say that 1/4 acre (10,890 square feet) is the minimum at $29.99, so any area less than that gets the minimum price, but anything over can go in tenths of an acre (1089 square feet) at $6 per tenth additional.

4) Fertilizing price calculation - 1/4 acre minimum at $39.99, $10 per tenth thereafter.

5) Weed control price calculation - 1/4 acre minimum at $30.99, $8 per tenth thereafter.

6) Pest Control price calculation - 1/4 acre minimum at $34.99, $9 per tenth thereafter.

7) Package pricing - combine the above, any two gets a 5% discount, any 3 gets a 10% discount, all gets a 12% discount.

You do not have to include this, but these are the functions I used for my previous code to give an idea:

void DisplayMenu(void);  //DisplayMenu Function Prototype
void ConvertArea(void);  //ConvertArea Function Prototype
void CalculateMowPrice(void);  //CaclulateMowPrice Function Prototype
void CaclulateFertilizePrice(void);  //CalculateFertilizePrice Function Prototype
void CalculateWeedPrice(void);  //CalculateWeedPrice Function Prototype
void CalculatePestPrice(void);  //CalculatePestPrice Function Prototype
void CalculatePackagePrice(void);  //CalculatePackagePrice Function Prototype
int CalculateSalesUnits(float squareFeet);  //CalculateSalesUnits Function Prototype