Below is an instruction that describes a task. Write a response that appropriately completes the request in Thai. Please answer in Thai.

### Instruction:
Rookie police officers have to buy duty shoes at the full price of $85, but officers who have served at least a year get a 20% discount. Officers who have served at least three years get an additional 25% off the discounted price. How much does an officer who has served at least three years have to pay for shoes?

### Response:
 ก่อนอื่นเราจะหาราคาหลังส่วนลดแรก: 85 - (85 * 0.2) = 68 เหรียญ
จากนั้นเราจะหาราคาหลังส่วนลดที่สอง: 68 - (68 * 0.25) = 51 เหรียญ