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

### Instruction:
A store is selling mobile phones. This store contains 20 units of phones but the owner found out that there were 5 defective units. After he got rid of the defective units, he started to sell the rest of his units to customer A who bought 3 units, customer B who bought a certain amount of units, and customer C who bought 7 units. In the end, all non-defective units were sold. How many units were sold to Customer B?

### Response:
 มีโทรศัพท์มือถือที่ไม่เสียเปรียบ 20 - 5 = <<20-5=15>>15 หน่วย
เนื่องจากลูกค้า A และ C ซื้อทั้งหมด 3 + 7 = 10 หน่วย ดังนั้นลูกค้า B ซื้อ 15 - 10 = 5 หน่วย