X is an unmarried dependent child who works part-time. During 2018, he earns $4,800 and has no other income. Must he file a return?

Answer: No. He had earned income only, and his gross income is less than his standard deduction of $6,500. However, if income taxes were withheld from his earnings, he'd want to file a return to get a refund of any income taxes withheld.

Please explain where they got the number.