Here is the full list of all 16 The Answer Is 42 achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Finish all levels in the game
Complete Ultimate question chapter
Find the Portal - to a new adventure??
Find the fish, thanks for it
Find the windmill
Find the rocket
Find a big star
Complete Deep thoughts chapter
Complete How many roads chapter
Find hitchhiker's thumb
Find the big answer
Find the Scorpio Constellation
Find the Ursa Major Constellation
Complete Time is an illusion chapter
Welcome to the journey
Complete The Guide chapter