.
The condition says that $360 is 20% less than the original price at which Mile bought the bike somewhen earlier.
So, your equation is 
(1-0.2)*x = 360,  or   0.8x = 360,
where x is the original price for Mike.  It implies that
x =  = 450.
Answer.  The cost orice of the bike was  $450.
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There is a bunch of lessons on percentage problems 
    - Percentage problems
    - Percentage word problems (Type 1 problems, Finding the Part)
    - Percentage word problems (Type 2 problems, Finding the Rate)
    - Percentage word problems (Type 3 problems, Finding the Base)
    - More complicated percentage problems
    - Problems on percentage that lead to unexpected results
in this site.