Void staff + Oblivion orb combo. How it is counted? Void staff = 40% magic pen.

Oblivion orb = 15 magic pen (flat).

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First scenario:

We have opponent with 50 magic resist. First we add void staff magic pen there (40%), which lefts us with 30 magic resist. Then we add 15 flat magic pen from oblivion orb, which leaves our opponent with 15 magic resist left. 

Second scenario:

We have opponent with 50 magic resist. First we add oblivion orb magic pen there (15 flat), which lefts us with 35 magic resist. Then we add void staff (40%) magic pen there, which leaves our opponent with  21 magic resist left.

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Question is, which magic pen adds up first?

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If it works like the second scenario, it's never worth to buy oblivion orb.