12 workers can do a peice of work in 20 days. How many workers should be added to complete work in 16 days?​

3 workers

if 12 workers can do a piece of work in 20 days, then one person can do it in 240 days. 240/16 is 15. That is the number of workers that you need in total to it in 16 days. Since the question ask how many more workers,
15 - 12 = 3 more workers.