12 men working 8 hours a day build a 60 m long wall in 15 days. How many days will 18 men working 10 hours a day take to build a 90 m long wall of the same kind?

The first wall takes $12\times 8\times 15=1440$ man-hours for 60 m, i.e. 24 man-hours per metre. A 90 m wall needs $90\times 24=2160$ man-hours, and 18 men working 10 hours supply 180 man-hours a day, so it takes 2160/180 = 12 days. (Forgetting that the second wall is half as long again gives 1440/180 = 8 days.)