(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=20 16000/50+32400/50-1800х/50-480=20 320+648-36х-480=20 -36х=20-320-648+480 -36х=-468 х=468*36 х=13