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Answer (D) is correct . The average collection period is the average time it takes to receive payment from customers. Because one-half of the customers will pay on day 10 and half will pay on day 30, the average collection period is 20 days [.5(10 days) + .5(30 days)].

Answer (A) is incorrect because Assuming half of the customers pay on day 10 but ignoring the remaining customers results in 5 days.
Answer (B) is incorrect because Assuming all customers take the discount results in 10 days.
Answer (C) is incorrect because The amount of 15 days assumes half of the customers pay on day 30 but ignores the remaining half of the customers who pay on day 10.