Mr. Jamal employed 50 workers to finish a work within 30 days. After 20 days he found out that only 50% of the work had been completed. How many additional workers will Mr. Jamal need to finish the work in time?

50 workers in 20 days can do 1/2 of the job

1 worker in 1 day can do 1/(2*50*20) of the job

                                   = 0.0005 of the job

Now,

0.0005 job can be done in 1 day by 1 worker

1/2 job can be done in 10 day by 1*0.50/(10*0.0005) worker

                                              = 100 workers

So, additional worker needed = (100 - 50) workers = 50 workers