Answer:
3 workers
Step-by-step explanation:
you currently have 100 employees that work during 49 weeks per year (= 52 weeks - 3 vacation weeks), that gives you a total of 4,900 work weeks.
since you want to offer your employees one additional week (4 in total), you must replace 100 work weeks with extra employees:
100 work weeks / 48 weeks per worker = 2.083 ≈ 3 workers, if you hire them full time.