1. The problem is to calculate the total work done when 15 workers work for 28 days. 2. The formula used here is: $$\text{Work} = \text{Number of workers} \times \text{Number of days}$$ 3. This formula works because work done is directly proportional to both the number of workers and the number of days they work. More workers or more days means more total work. 4. Substituting the values: $$\text{Work} = 15 \times 28$$ 5. Multiplying these gives: $$\text{Work} = 420$$ labor-days. 6. The unit "labor-days" means the amount of work done by one worker in one day. So 420 labor-days means the total work done is equivalent to one worker working 420 days, or 15 workers working 28 days. This explains why the work is calculated as $$15 \times 28 = 420$$ labor-days.