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This problem trains for: SAT-I, AMC-8, GMAT.

10 dogs are digging up bones. They work for 12 days, 8 hours a day. How many days do 16 dogs need to dig up the same number of bones if they work only 6 hours a day?

The amount of work needed is fixed, since they have to dig the same number of bones. The more hours per day they work, the less days they will need to finish the task. Therefore, the number of hours and the number of days are inversely proportional quantities. Also, the more dogs there are, the less days and hours they need. Therefore, the number of dogs is also inversely proportional with the number of hours and the number of days.

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They will need 10 days.