You want exactly nine gallons of water, but you have seven-gallon bucket and five-gallon bucket. To solve this problem, we use the method of selective combination.
We should fill the seven gallon bucket and then pour it into five gallon bucket, leaving the two gallon behind. Discard five gallons and pour two gallon into five-gallon bucket and refill seven-gallon bucket. You will have a total of nine gallons.
To elaborate, the selective combination is an insight producing process that occurs when one merges two or more components that are seemingly unrelated to produce a working solution.
In this case, two components combined are the five-gallon bucket and the seven gallon bucket.
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