The amount of markup was found to be M = $530.40, but

The amount of markup was found to be 
M = $530.40,
 but we need to find the percent markup, 
%MCOST.
 The markup amount will be compared to the price the wholesaler paid using the following equation. Note that the percent markup found will be a decimal and must then be converted to a percent.
%MCOST = 
M
C
The wholesaler paid $680 for the guitar, so we have 
C = $680.
 Substitute the values for M and C into the equation.
%MCOST
 = 
M
C
 
 = 
530.40
 
 = 
This is the decimal value for the markup rate so it needs to be multiplied by 100% to find the percent markup. Thus, the percent markup is 
 %.
The cost of the watch to the wholesaler is $90 and we determined that as a decimal, 
100% + %MCOST = 1.35.
Substitute the cost of the watch and the decimal form of 
100% + %MCOST
 into the equation to find the selling price of the watch (in $).
SP
 = 
C(100% + %MCOST)
 = 
90(135%)
 = 
90
  
 
 = 

The wholesale cost of the birdcage was given to be $55, so we have 
C = $55.
 We determined that as a decimal, 
100% − %MSP = 0.53.
Substitute these values into the equation to find the original selling price (in $) of the birdcage, rounding the result to the nearest cent.
SP0
 = 
C
100% − %MSP
 
 = 
$55
 
 
 = 
$