Answer:
40,000 workers
Explanation:
This question deals with simple rates proportion.
If it takes 20 years for X workers to complete a job,
if we double the amount of workers, we will take half the time.
hence to halve the time from 20 years to 10 years, we will need to double the amount of workers.
In this case we will need 20,000 workers x 2 = 40,000 workers