10100 is a binary number which is equal to 20
in decimal .
See the following procedures on how to convert  a 
binary  to a decimal  number.
 

  1 X 2^n-1 = 1 x 2^5-1 = 1 x 2⁴   =  16  

  0 X 2^n-2 = 0 x 2^5-2 = 0 x 2³  =   0

  1 X 2^n-3 = 1 x 2^5-3 = 1 x 2²  =   4

  0 X 2^n-4 = 0 x 2^5-4 = 0 x 2¹  =   0

  0 X 2^n-5 = 0 x 2^5-5 = 0 x 2⁰  =   0 
  _____________________________________________
                                                    20
             

20 is a decimal number which is equal to10100 binary
See the following procedures on how to convert a  
decimal to a binary  number. 

The largest power of 2 that fits on 20 is 2⁴
  1 x 2⁴  =  16   fits  20-16=4
  0 x 2³  =   0   does not fit 8>4
  1 x 2²  =   4   fits   4 - 4=0
  0 x 2¹  =   0 does not fit 
  0 x 2⁰  =   0 does not fit

_________________________

         10100