Question about enchantment extensions

I play a monk and I was wondering if anyone knows how the enchantment extensions stack work.

I use a weapon that has enchantment last 20% longer.
I was thinking about using Blessed Aura my divine favor is 12 so my monk enchantment would last 30% longer.

Does anyone know if they stack or if would be negated?
Also if they stack do they stack at the same time or one then the other?

They stack, but I'm not sure they stack at the same time...

I always thought that logically it would be like this...

The spell has the normal duration, let's say 10 seconds.
Then the 20% on the weapon kicks in and you get another 2 seconds, making it 12.
Then the 30% kicks in, adding another 3.6 seconds, GW rounds up, so 16 seconds total.

That's how I thought it would be, logically, but people keep telling me it would be 50% longer if you have a 20% weapon and 30% from blessed aura..

Someone wanna clarify?