Weights to Measure from 1 to 40 Kg

How can you measure  any weight between  1Kg  to 40Kg  using upto four weight without repeating any weight ?

  
Answer is  1Kg ,3Kg ,9Kg  ,27Kg 


With these 4 weights you can find any weight from 1 to 40  by  adding or subtracting

Is it not interesting !!!!!!!