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Answer:
7 1/3 hours
Explanation:
We are to assume that the total number of heater-hours required is a constant. That is, the time is inversely proportional to the number of heaters.
If the number of heaters goes up by a factor of 6/4 = 3/2, then the number of hours will go down by the inverse factor: 2/3.
With 6 heaters instead of 4, the time required is (2/3)(11 hours) = 7 1/3 hours.