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Maybe you get it, but I can't tell from your choice of words. I'm not sure about the sequential part.

I would say it differently: The values passed from the main() method to the...

If you understood why, you wouldn't have to ask, but try it and see what happens. Then figure out why. If you can't, ask.

Oh, wait a minute. You did ask.

In the method calcAverageBill(), the...

You're not using the values passed to the CO2 methods, but creating empty/null values to use instead:

public double calcAverageBill(ArrayList<Double> monthlyBill)
{
// **** insert...