There are a couple of ways you can go at this. One way is to look at the price multipliers. For a markdown of d, the price is multiplied by 1-d. For a markup of u, the price is multiplied by 1+u. Your original price is multiplied by ...
35% markdown: (1 - 0.35) = 0.65
25% markdown: (1 - 0.25) = 0.75
18% markup: (1 + 0.18) = 1.18
15% markdown: (1 - 0.15) = 0.85
The product of all these multipliers is
0.65×0.75×1.18×0.85 = 0.4889625
As you note, $5600×0.4889625 = $2738.19.
Since this multiplier is 1-d, we can find d as
1 - d = 0.4889625
d = 1 - 0.4889625 = 0.5110375
The markdown percentage is this number times 100%:
markdown% ≈ 51.1%
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Alternatively, you can work with your final value to find the percentage change from the original.
percent change = ((final value) - (original value))/(original value) × 100%
= (2738.19 - 5600)/5600 × 100%
= -2861.81/5600 × 100%
= -0.5110375 × 100% . . . . . . . this should look familiar
percent change ≈ -51.1%
When the change is negative, it is a markdown (not a markup).
The markdown is 51.1%.
