(16000+9*(900-50x)*4)/50-80*6=20

(16000+9*(900-50х)*4)/50-80*6=20((16000+(8100-450х)*4)/50-480=20(16000+32400-1800х)/50-480=2016000/50+32400/50-1800х/50-480=20320+648-36х-480=20-36х=20-320-648+480-36х=-468х=468*36х=13