Samantha wants to buy a new book that costs $18. She decides to save money by doing household chores for her neighbors. She earns $6 per hour for cleaning and $4 per hour for gardening. If she spends 2 hours cleaning and 3 hours gardening each weekend, how many weekends will it take her to save enough money to buy the book? Let's think step by step and output the final answer within \boxed{}.
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