Come up with a question and reasoning that would justify this answer: 5
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The question is: 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?
Step-by-step reasoning process: There were 20 - 5 = 15 units of non - defective smartphones. There were 3 + 7 = 10 units sold to customers A and C. So, 15 - 10 = 5 units were sold to customer B.

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