Question 1154714
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They take 12 hours to do the job together.<br>
In the beginning, they work together for 4 hours.  The fraction of the job that gets done is 4/12 = 1/3; 2/3 of the job remains.<br>
Edward takes 18 hours to do the job alone.  To do the 2/3 of the job that remains alone, the required amount of time is 2/3 of 18 hours, or 12 hours.<br>