isentropic mass taken for calculation of area of nozzle

engnr_arsalan

i was reading an example of area calculation of a nozzle,in which mass flow rate was and coeff. of discharge (ratio of actual mass flow rate to isentropic mass flowrate) was given.
i though the mass flow rate given in example is isentropic and we will first calculate actual mass flowrate from co.eff of discharge then area of nozzle..but totally opposite was done in example the mass flowrate given was actual and they calculated isentropic mass from it then area of nozzle..i didnt understand why they took isentropic mass..because design problems are based on actual condition..
kindly correct me if i am wrong..