From 7: 30 am to 4: 30 pm, a * * * is:

4: 30 pm = 65438+6: 30 pm

16: 30-7: 30 = 9 (hours)

Subtract 1 hour at noon:

9- 1 = 8 (hours)

Then calculate the average number of parts manufactured by the main workers per hour:

56 ÷ 8 = 7 (piece)