Carlo alone can do a job in 10 days. With Dina's help, the job takes 6 days. How long would Dina take alone?
Correct answer: B. 15 days
Together they do 1/6 per day and Carlo does 1/10. Dina's rate = 1/6 − 1/10 = 5/30 − 3/30 = 2/30 = 1/15, so she would take 15 days alone.
hard