Let the original bill be 
, then the final bill after the charges been added are 
, where 
is the increase in the bill by 0.2 (100%+20% = 120% = 1.2)
The range of the original bill is
200 < < 800, subtract each term by 25
175 <
< 775, divide each term by
145.83 < < 645.83
Hence, the range of the original bill is between $145.83 and $645.83
The range of the original bill is
200 <
175 <
145.83 <
Hence, the range of the original bill is between $145.83 and $645.83