10×x=8×25
10×x=200
x=200:10
x=20
x:20=4800:80
x:20=60
x=60×20
x=1200
(x-380)×50=2000
x-380=2000:50
x-380=40
x=380+405
x=420
(x+8)-70=4
x+8=70×4
x+8=280
x=280:8
x=35
640:(25-x)=40
25-x=640:40
25-x=16
x=25-16
x=9
2700:(x-12)=90
x-12=2700:90
x-12=30
x=30+12
x=42