よくぞ

adverb

how admirably; how wonderfully; how skillfully

Polite, emphatic adverb expressing admiration or praise for an action. Often used in exclamatory or appreciative remarks. The ぞ adds a slightly formal or old-fashioned polite emphasis.

• よくぞここまで成せい長ちょうしてくれました。

  How wonderfully you have grown up to this point.

• よくぞその難なん問もんを解ときましたね。

  How admirably you solved that difficult problem.