Mana cost efficiency & Efficiency support gem

Hello Exiles,

I really dont understand what is the difference between Mana cost efficiency & Efficiency support gem.

So if I use Efficiency support gem (skills cost 40% less), a skill needing 100 mana will need 60.

If I have 40% mana cost efficiency, same skill will cost 60 mana again?

As I remember if I have 100% cost efficiency, skill will cost half of its original cost. But if lets say there is a support gem says "skills cost 100% less", it means mana cost will be 0?

What if someone has both? which applies first?

SOOOOO CONFUSED

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seaman#4993 wrote:

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Enoch#7385 wrote:

If I have 40% mana cost efficiency, same skill will cost 60 mana again?

100 / (1 + 0.4) ≈ 72

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But if lets say there is a support gem says "skills cost 100% less", it means mana cost will be 0?

Yes.

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What if someone has both? which applies first?

It doesn't matter
(100 * 0.6) / 1.4 = (100 / 1.4) * 0.6

Highly appriciate the help Sir. Thank you.