(39+x)-84=78
39+х-84=78
х=78-39+84
x=123
(x-83)+316=425
x-83+316=425
x=425+83-316
x=192
(600-x)-920=126
600-x-920=120
x=120+920-600
x=440
253-(X-459)=138
253-x-459=138
x=138+459-253
x=344
502-(217-x)=421
502-217-x=421
x=421+217-500
x= 135
871-(x+157)=385
871-x+157=385
1028-x=385
x=1028-385
x=643