This is an excellent question because it allows the student to demonstrate an understanding that (a) digits have different 'meaning' depending on the context - the 5 'means 500' and the 8 'means 80'; and (b) there is a difference between replying with a true statement and answering correctly.

Bobby has made a true statement that is related to the question, but hasn't completely answered the question. The correct is that the value of 5 in 582 is 500, and an even better answer would be "the value of 5 in 582 is 500 because the 5 is in the hundreds place".

This distinction is something that many teachers overlook. There is an excellent handbook for teachers called "Teach Like A Champion" [1] which emphasises that "Right is Right" - here is an excerpt:

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Right Is Right is about the difference between partially right and all-the-way right—between pretty good and 100 percent. The job of the teacher is to set a high standard for correctness: 100 percent. The likelihood is strong that students will stop striving when they hear the word right (or yes or some other proxy), so there's a real risk to naming as right that which is not truly and completely right. When you sign off and tell a student she is right, she must not be betrayed into thinking she can do something that she Cannot.

Many teachers respond to almost-correct answers their students give in class by rounding up. That is they'll affirm the student's answer and repeat it, adding some detail of their own to make it fully correct even though the student didn't provide (and may not recognize) the differentiating factor. Imagine a student who's asked at the beginning of Romeo and Juliet how the Capulets and Montagues get along. “They don't like each other,” the student might say, in an answer that most teachers would, I hope, want some elaboration on before they called it fully correct. “Right,” the teacher might reply. “They don't like each other, and they have been feuding for generations.” But of course the student hadn't included the additional detail. That's the “rounding up.” Sometimes the teacher will even give the student credit for the rounding up as if the student said what he did not and what she merely wished he'd said, as in, “Right, what Kiley said was that they don't like each other and have been feuding. Good work, Kiley.” Either way, the teacher has set a low standard for correctness and explicitly told the class that they can be right even when they are not. Just as important, she has crowded out students' own thinking, doing cognitive work that students could do themselves (e.g., “So, is this a recent thing? A temporary thing? Who can build on Kiley's answer?”).

When answers are almost correct, it's important to tell students that they're almost there, that you like what they've done so far, that they're closing in on the right answer, that they've done some good work or made a great start. You can repeat a student's answer back to him so he can listen for what's missing and further correct—for example, “You said the Capulets and the Montagues didn't get along.” Or you can wait or prod or encourage or cajole in other ways to tell students what still needs doing, ask who can help get the class all the way there until you get students all the way to a version of right that's rigorous enough to be college prep: “Kiley, you said the Capulets and the Montagues didn't get along. Does that really capture their relationship? Does that sound like what they'd say about each other?”

In holding out for right, you set the expectation that the questions you ask and their answers truly matter. You show that you believe your students are capable of getting answers as right as students anywhere else. You show the difference between the facile and the scholarly. This faith in the quality of a right answer sends a powerful message to your students that will guide them long after they have left your classroom.

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[1] http://www.amazon.com/gp/product/1118901851