200 * (х + 71) = 54 000
х + 71 = 54 000 : 200
х + 71 = 270
х = 270 - 71
х = 199
_____________
200 * (199 + 71) = 54 000
900 * (16 - с) = 5400
16 - с = 5400 : 900
16 - с = 6
с = 16 - 6
с = 10
___________
900 * (16 - 10) = 5400
400 * ( х - 12) = 3600
х - 12 = 3600 : 400
х - 12 = 9
х = 9 + 12
х = 21
_________
400 * (21 - 12) = 3600