Answer: 3
Step-by-step explanation:
Printers are said to be working at a constant rate with 6 printers working for 12 hours. This means in other to achieve a similar output with 1 printer;
12 hours × 6 = 72 hours
Total hours required to complete printing = 72 hours
Assuming adding more printers working at constant rate in other to reduce completion hours to 8
Total completion hours ÷ number of printers = 8 hours
72 ÷ number of printers = 8
Nunber of printers = 9
Therefore, number of printers that must be added = 9 - 6 = 3