How can I do the math specifically on this

The water rates vary depending on the type of usage. A code of H means home use, a code of C means commercial use, and a code of I means industrial use. Any other code value should be treated as an error. Water rates are computed as follows:
Code H: Code C:
Code I:
$5.00 plus $0.005 per gallon
$1000.00 for the first 4 million gallons used plus
$0.025 for each additional gallon used
$1000.00 if usage does not exceed 4 million gallons
$2000.00 is usage is between 4 million and 10 million gallons $3000.00 is 10 million gallons or more