Ken works on a job for 10 days and completes 40% of the job. Then he employs Jacob who is 50% as efficient as Ken to help him complete the job. In how many days was the task completed?

Explanation:

In 10 days, Ken had completed 40% of the job.

Therefore, he would complete 40\10 = 4% of the work in a day.

As Jacob is 50% as efficient as Ken, he would complete 2% of the work in a day.

Working together they will complete 4 + 2 = 6% of the work in a day.

To complete the balance 60% of the job, they will take 60\6 = 10 more days.

Hence, the task will be completed in 20 days.