Can you explain how to solve this math problem: Jamaar loves fresh fruit and is headed to the store with $10 he earned mowing lawns. Including tax, peaches and pears are $.5 each, apples are $.75 each, kiwis are $1, and plums cost $.25 each. If he has already purchased 3 peaches, 4 pears, 2 kiwis, and 5 apples, how many plums can he buy?