(31*550-X)*120=10200
( 31 *550 -х) *120 =10 200
31* 550-х =10 200 :120
17 050 - х=85
х =17 050 - 85
х =16 965
проверка :
( 31 * 550 - 16 965 )* 120 =10 200
17050-х=10200/120
17505-х=85
х=17050-85
х=16965