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1. Placing "each week" at the end of the sentence implies that Marsha needs to save $10 every single week in order to reach her goal of buying a new bike.
2. Placing "each week" after "new bike" suggests that Marsha's goal is to buy a new bike every week, and she will need to save $10 each time to achieve this.
3. Placing "each week" before "Marshall" indicates that Marsha needs to save $10 each week specifically in order to reach her goal of buying a new bike.