[Skunkworks] Calculation of basic pay given the net pay

Hi Skunks, This problem is spinning my head yet i am sure someone may have done it. Company X netiates with an employee a net pay earning. They want to calculate backwards the gross pay based on the net pay. The deductions affecting the netpay are NSSF (200), NHIF(320)and the PAYE. How can i determine the PAYE and then the gross pay? Regards Shadrack