The efficiency ratio is (1+20%): 1 = 6: 5.
The original time is 1÷(6-5)×6=6 (hours).
If the work efficiency is improved by 25% after processing 120 pieces according to the original plan, it can be completed 40 minutes in advance.
Then the efficiency ratio is (1+25%): 1 = 5: 4.
The time ratio is 4: 5.
After the work efficiency is improved by 25%, it takes 40 hours (1-4/5) = 200 (minutes) = 10/3 hours.
The original work efficiency is 120 ÷ (6- 10/3) = 45 (per hour).
The original planned output of parts is 45×6=270 (pieces).