A road of 5 km length will be constructed in 100 days. So 280 workers were employed. But after 80 days it was found that only $\frac{7}{2}$ km road was completed. Now how many more people were need to finish the work in the specified time?

280 persons worked 80 days to complete 3500 meters road. 

So that a person can complete the work perday is $\frac{3500}{280·80}=0.15625$

So to complete $1500$ meters, we need $\frac{1500}{0.15625}=9600$ man days

to complete the abvoe task in 20 days, we need to get the people $\frac{9600}{20}=480$