There were 20 - 5 = <<20-5=15>>15 units of non-defective smartphones.

There were 3 + 7 = <<3+7=10>>10 units sold to customers A and C.

So, 15 - 10 = <<15-10=5>>5 units were sold to customer B.

# Answer

5
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
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?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
5
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?
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