Question 1172530
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Theresa working alone for 3 days completes 3/15 = 1/5 of the job; 4/5 of the job remains to be done.<br>
Theresa and Rowena together complete those 4/5 of the job in 4 days; that means together they complete 1/5 of the job each day.<br>
Each day Theresa alone does 1/15 of the job and together they do 1/5 of the job.  That means each day Rowena does 1/5-1/15 = 3/15-1/15 = 2/15 of the job.<br>
Since Rowena does 2/15 of the job each day, the number of days it would take Rowena to do the whole job alone is 15/2, or 7 1/2.<br>
ANSWER: 7 and a half days<br>