①x+40%x=42 
       1.4x=42
  1.4x÷1.4=42÷1.4
          x=30
    ②x-25%x=15
       0.75x=15
 0.75x÷0.75=15÷0.75
           x=20
   ③x-65%x=0.7  
      0.35x=0.7
0.35x÷0.35=0.7÷0.35
          x=2
   ④x+75%x=3.5
      1.75x=3.5
1.75x÷1.75=3.5÷1.75
          x=2