Ella can finish a job in 12 days. She works for 4 days, and Fred finishes the rest in 10 days. How long would Fred take to do the whole job alone?
Correct answer: B. 15 days
In 4 days Ella completes 4/12 = 1/3 of the job, leaving 2/3. Fred does 2/3 in 10 days, so the whole job would take him 10 ÷ 2/3 = 15 days.
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