Ответ:

75-66:(4+30:(7+56:(75-x)))=120:30

75-66:(4+30:(7+56:(75-x)))=4

75-4=66:(4+30:(7+56:(75-x)))

66:(4+30:(7+56:(75-x)))=71

66:71=4+30:(7+56:(75-x))

66/71-4=30:(7+56:(75-x))

-218/71=30:(7+56:(75-x))

30:(-218/71)=7+56:(75-x)

30*(-71/218)=7+56:(75-x)

-1065/109=7+56:(75-x)

-1076/109-7=56:(75-x)

-1839/109=56:(75-x)

56:(-1839/109)=75-x

56*(-109/1839)=75-x

-6104/1839=75-x

x=75+6104/1839

x=75+3 587/1839

x=78 587/1839