We have 6 workers the first time and 9 workers the second time.

The least common multiple of 6 workers and 9 workers is 18 workers.

if it takes 6 days for 6 workers to build a garage, then what about 18 workers?
They would work 3 times as fast so it would take them only 1/3 as long as it
took 6 workers, or only 2 hours.

So if it takes 18 workers 2 hours to build a garage, then 9 workers would take
twice as long, so it would take them 4 hours.

Edwin

    Yes, you can use the method these two gentlemen propose you.


    But it is the same, as to fly from Chicago  to Indianapolis with intermediate landing in Denver.


    The way I propose in my post is the standard direct straightforward shortest Algebra way for this type of problems.