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If a writer upvotes a reply with 100 %age and another reply with 50% ( there is 30 steems bountry), the former gets 20 steems bountry and the latter gets 10 steems bountry. right?

Yes you are getting it. But keep in mind that the community votes with voting power (rshares) and the bounty creator votes with a %-age slider.

This is to give the creator and easy way to distribute most of the bounty and avoid abuse by the community.

Thanks a LOT.
Have a wonderful weekend ~🌵