Dreya is saving money to purchase a $900 computer, and she saves $10 the first week. Each week after that, she saves $3 more than the previous week, except in the last week, when she reaches $900. To have a final sum of exactly $900, how much money does Dreya need to save in her final week of saving?

Before the final week, Dreya’s weekly savings follow 10, 13, 16, and so on, increasing by $3 each week. The total for 21 such weeks is 21/2 × (10 + 70) = $840, since the 21st regular amount is $70. She needs $900 − $840 = $60 in the final week, so the correct choice is $60.

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