Answer:

While I AGREE with the other answers giving 9.6, so, realistically, 10 men to complete the job in 5 days, I fail to see where, in the question, the asked specifies that the job is to be completed within the same 5 day time frame; therefore, my answer is slightly different.

It's established that the job takes 480 hours to complete (12 × 8 × 5 = 480); however, as I stated above, it is assumed the builder wishes to stay within the 5 day time frame … but, they may not … so, for the sake of choices, I want to GO over the other answers:

Possible answer #1: 1 worker will complete the job in 48 days;

Possiblity #2: 2 workers will complete the job in 24 days;

Possibility #3: 3 workers will complete the job in 16 days;

Possibility #4: 4 workers will complete the job in 12 days;

Possibility #5: 5 workers will complete the job in 9.6 days;

Possibility #6: 6 workers will complete the job in 8 days;

Possibility #7: 7 workers will complete the job in (approximately) 6.85 days;

Possibility #8: 8 workers will complete the job in 6 days;

Possibility #9: 9 workers will complete the job in (approximately) 5.33 days;

Possibility #10: 10 workers will complete the job in 4.8 days;

Possibility #11: 11 workers will complete the job in 4.36 days;

Possibility #12: 12 workers will complete the job in 4 days;

Possibility #13 : 13 workers will complete the job in (approximately) 3.6 days;

Possibility #14: 14 workers will complete the job in (approximately) 3.4 days;

Possibility #15: 15 workers will complete the job in 3.2 days;

Possibility #16: 16 workers will complete the job in 3 days;

Possibility #17: 17 workers will complete the job in (approximately) 2.8 days;

Possibility #18: 18 workers will complete the job in (approximately) 2.6 days;

Possibility #19: 19 workers will complete the job in (approximately) 2.5 days;

Possibility #20: 20 workers will complete the job in 2.4 days;

I could keep going, until I REACH one day, but I believe this adequately demonstrates the numerous answers to this question.