Nostalrius Begins - Quality wow vanilla realm (1.12)

I've looked everywhere and couldnt find a clear answer to the question i have:

in the spec Dual Wield Specialization (5/5 points) it says:

Increase the damage done by your offhand weapon by 25%.

I know that my offhand weapon deals only 50% of the normal damage by default, but does it now with the spec get increased to 75% (0.50+0.25) or just to 62.5% (0.50*1.25)? or is there another calculation?

Thanks for the answer in advance!

It goes to 62,5% of normal damage.

Ok thanks! I think the wording isn't clear enough for new people to know, but it helps to know, otherwise I would have maybe skilled differently. Thanks again

Rule of thumb I find when it comes to wow descriptions is if it doesn't use the word 'additional', it's multiplied.