A rope pulley having a mean diameter of \(1.5 \mathrm{~m}\) rotates at \(90 \mathrm{rpm}\); angle of lap of ropes \(=170^{\circ}\); angle of groove \(=45^{\circ}\), safe tension per rope \(=750 \mathrm{~N}\), and coefficient of friction between the ropes and sides of groove \(=0.25\). Calculate the number of ropes for transmitting \(50 \mathrm{~kW}\).