Eric took 20 hours to do the job alone.
Step-by-step explanation:
Given :
The amount of work Joe alone can do per hour = 75/5 = 15 packages.
The amount of work Joe and Eric together do per hour = 75/4 = 18.75
The amount of work Eric alone can do per hour = 18.75 - 15 = 3.75
The time Eric took to complete the entire work = 75 / 3.75 = 20 hours.