30 workers can build 60 machines working 4 hours per day. How many extra workers are required to produce 90 machines working 6 hours per day?

Question: 30 workers can build 60 machines working 4 hours per day. How many extra workers are required to produce 90 machines working 6 hours per day?

Solution:
4 hours to build 60 machines by 30 workers
1 hour to build 1 machine by = (30 × 4)/60 workers
6 hours to build 90 machines by = (2 × 90)/6 workers
= 30 workers

∴ extra workers = (30 - 30) = 0 workers