Multiple choice

A and B can do a job in 10 days and 5 days, respectively. They worked together for two days, after which B was replaced by C and the work was finished in the next three days. How long will C alone take to finish 60% of the job?

  1. 18 days

  2. 30 days

  3. 25 days

  4. 24 days
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A's rate = 1/10 job/day, B's rate = 1/5 job/day. Together they do 3/10 per day. In 2 days: 6/10 = 3/5 done. Remaining: 2/5. A + C do 2/5 in 3 days, so A+C rate = 2/15 per day. C's rate = 2/15 - 1/10 = 1/30 job/day. C alone needs 30 days for full job, so 60% takes 18 days.

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