A dose 190 cGy is prescribed at the midplane of the arm. Parallel opposed cobalt-60 beams are used. The field size is 6×12 cm2 , and the SAD is 80 cm. The air dose calibration factor - ADR is 112.6 cGy/min at 80 cm for 10×10 cm2 field, and the field size dependence factor for 6×12 cm2 field is 0.995. The arm is 9 cm in diameter. The isocenter is at middepth in the arm. What is the required treatment time for each field?