Three dogs need 7 hours to dig 9 holes. Five birds spend 40 minutes to build 2 nests. Maintaining these rates, how many more minutes does a dog take to dig a hole than a bird takes to build a nest? 

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Solution

Since 1/3 of the dogs do 1/3 of the work in the same period, we can conclude that $3 / 3=1$ dog needs 7 hours to dig $9 / 3=3$ holes. Working $1 / 3$ of the time, this dog will do $1 / 3$ of the work, so we can ensure that 1 dog spends $(7 \cdot 60) / 3=140$ minutes to dig one hole. Similarly, 1 bird needs 40 minutes to build $2 / 5$ of a nest and $5 / 2 \cdot 40=100$ minutes to build $5 / 2 \cdot 2 / 5=1$ nest. Therefore, to dig a hole, a dog spends 40 minutes more than a bird to build a nest.