Suruma herdai ma dekheko mistake
Call garda yasto chha   callone.Calculatecost();No argument passed here.

mathi defined gareko function ma Argument lirako chha.Fix this.


Double CalculateCost(double min) 
        {
            totalcost = min * unitcost;
            return totalcost;
        }

for example tyo method lai yasari declare gara.sabai method lai public declare gara.Normally methods should be public and attribute as private.

public Double CalculateCost()
        {
            totalcost = min * unitcost;
            return totalcost;
        }


Tespachhi thik hunchha.