Remember that the -ed past simple verb ending usually has a silent “e”, so it doesn’t add a syllable. Because of the /k/ sound, the “d” becomes unvoiced – /t/. Curiously, the “t” of “it” can be voiced, making it sound like a “d”. This is optional. The final “t” can be pronounced with the tongue, or down where you make a /h/, but isn’t released.
To work something out is to solve a puzzle, mystery or problem.