First of all you count how many hours they together work a day.
20x8=160
They work 160 hours a day.
Then you need to find out how many hours they work during the whole building process.
160x54=8640
The building takes 8640 hours to build.
Then you need to find out how many hours it takes for 18 men to build this with just 7,5 hours a day.
7,5x18=135
To find out how many days it takes to build it:
8640/135=64
So it takes 64 days.
64-54=10
So it takes 10 more days.
—
I hope that helps!
And sorry if it was confusing, english isn’t my first language...