6. A mall offers discounts to customers, stipulating:
(1) If a single purchase does not exceed 200 yuan, no discount is given;
(2) If a single purchase exceeds 200 yuan but does not exceed 500 yuan, a 10% discount is applied to the marked price;
(3) If a single purchase exceeds 500 yuan, the first 500 yuan is discounted according to rule (2), and the amount exceeding 500 yuan is given an 80% discount.

A person went shopping twice, paying 168 yuan and 423 yuan respectively. If he had purchased the same items in one trip, the amount he should pay is ( ).
(A) 522.8 yuan
(B) 510.4 yuan
(C) 560.4 yuan
(D) 472.8 yuan
6. (C)

Obviously, 168 is less than $200 \times 0.9=180$, so it has not been discounted; 423 is less than $500 \times 0.9=450$, and greater than 200, so this is the price after a 10% discount; together, it is $168+423 \div 0.9=638>$ 500, according to (3), the amount to be paid is
$$
500 \times 0.9+138 \times 0.8=560.4 \text { yuan. }
$$