Karim can do a job in 15 minutes and his brother takes twice as long to do the same job. If they work together, how long it takes to complete the job?

Karim can do a job in 15 minutes and his brother takes twice as long to do the same job. If they work together, how long it takes to complete the job? Correct Answer 10

If Karim takes 15 minutes then his brother takes 2 x 15 = 30 minutes to do the job. Together they can complete 1/15 + 1/30 = 3/30 = 1/10 of the job in a minute. So, together they can complete the job in 10 minutes.
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