Answer:
Basically, you have to think: more bricks : more time
more workers : less time
2,400 bricks 6 workers takes 18 hours
You now have to solve for 4,500 blocks and 10 workers
4,500 / 2,400 = 1.875 (times greater)
10 / 6 = 1.666666666 (times less)
So, we get 18 hours, multiply it by 1.875 and divide it by 1.666666666
which equals 20.25 hours
So, it seems you were correct on your third try.
Explanation: