(1)5x+35=650, 5x+35﹣35=650﹣35, 5x=615, x=123; (2)720÷3x=80, 720÷3x×3x=80×3x, 240x=720, x=3; (3)3.6x﹣0.9x=8.1, (3.6﹣0.9)x=8.1, 2.7x=8.1, x=3;