In the first sentence, "each week" is placed at the end of the sentence, indicating that Marsha needs to save $10 every week in order to achieve her goal of buying a new bike. This emphasizes the frequency of her saving.
In the second sentence, "each week" is placed after "buying a new bike", suggesting that Marsha needs to save $10 each week specifically for the purpose of buying a new bike. This placement highlights the intention behind her saving, which is to reach the goal of purchasing a new bike.
Overall, the first sentence focuses on the regularity of saving, while the second sentence emphasizes the purpose or destination of the savings. The placement of the italicized phrase changes the emphasis and tone of the sentence, showcasing different aspects of Marsha's goal and saving habits.