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Let x= daughters time in hours. 1/x = part of the job daughter can do in 1 hr.
mother's time = x-6. 1/x-6 = part of the job mother can do in 1 hr.
Job's time = x/3. 3/x = part of the job that can be done in 1 hr.
1/x + 1/x-6 = 3/x LCD=x(x-6)
x-6 + x = 3(x-6)
12-6=6 hrs mother's time.
1/12 + 1/6 = 3/12 = 1/4 of the job in 1 hr. Job takes 4 hrs which is 1/3 the time it would take the daughter working alone.