12 men will take 54 days to complete a building project.

1. Find the total work in man-days and divide by the new number of men.

For the same building project,
$\text{men} \times \text{days} = \text{constant}.$
$12 \times 54 = 9 \times d$
$d = \frac{12 \times 54}{9} = 72$

1. Find the time taken when 6 more men are added to the original team.

Number of men $=12+6=18$

Total work $=12\times54=648$ man-days

Time taken by $18$ men $=\dfrac{648}{18}=36$ days

2. Find the number of days saved compared with the original time.

Days saved $=54-36=18$ days