.
Mr. Downey can build a wall in 9 hours. His son Carl can build the same wall in 18 hours.
Mr. Downey started to build the wall, worked for 3 hours, and then stopped working.
How many hours would Carl require to complete the wall?
~~~~~~~~~~~~~~~~~
Working for 3 hours, Mr. Downey completed 1/3 of the job; 2/3 of the job remained.
Hence, his son Carl needs 2/3 of 18 hours to complete the remaining job.
ANSWER. Carl needs 12 hours to complete the job.
Solved.