Fiona is going to the grocery store for watermelons and pineapples. Watermelons cost $7 each and pineapples cost $4 each. Fiona has a total of $30 to spend. Use the equation 7w + 4p = 30 to answer the question below. If Fiona buys 3 watermelons, what is the maximum number of pineapples she can buy?