Depends entirely on what method you're using. Since you're using a Purchases account, it sounds like you may be doing the periodic method, something not used much anymore. It also sounds like you may be referring to doing an end-of-period adjusting entry. But you've pulled this out of context, so I'm not sure.
It also sounds a bit like a homework question. (The way it is worded is not how most people talk.) If it is, first see our guidelines about posting homework:
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