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Alan can build a brick wall in 9 hours.
Carl can build the same wall in 18 hours.
Mr. Alan started to build the wall, worked for 3 hours, and then stopped working.
How many hours would Carl require to complete the wall?
Let c = no. of hrs to complete the wall by Carl
Let the completed wall = 1
Each will do a fraction of the wall, the two fractions add up to 1
Multiply equation b;y 18 to clear the denominators
2(3) + c = 18
c = 18 - 6
c = 12 hrs to complete the wall