A beekeeper has a marginal benefit of MB-25 per hive. The marginal cost she has per hive is 5 for the first hive, 15 for the second, 25 for the third, 35 for the fourth, 45 for the fifth, 55 for the sixth, 65 for the seventh, 75 for the eighth, and so on for each subsequent hive. In addition, a nearby almond grower also has a marginal benefit of 15 per hive (for every hive). What is the efficient number of hives?