Answer:
12 hours
Step-by-step explanation:
For problems like this, it works well to think in terms of jobs per hour, rather than the given hours per job.
When you both work together, your rate is 1/3 job per hour. When you work alone, your rate is 1/4 job per hour. Then the rate increase that is contributed by your sister is the difference of these rates:
(1/3 job/h) - (1/4 job/h) = (1/3 -1/4) job/h = 1/12 job/h
If your sister can do 1/12 of the job in 1 hour, then it takes her 12 hours to do the entire job by herself.
