Problem 330

2. A certain product is divided into 10 quality levels. Producing the lowest level product yields a profit of 8 yuan per piece, and for each level increase, the profit per piece increases by 2 yuan. Using the same working hours, the lowest level product can produce 60 pieces per day, and for each level increase, the production decreases by 3 pieces. If the product that yields the maximum profit is the $k$-th level (the lowest level is the first level, and the levels increase with quality), then, $k$ equals: