In considering materiality for planning purposes, an auditor believes that misstatements aggregating $10,000 will have a material effect on an entity's income statement, but that misstatements will have to aggregate $20,000 to materially affect the balance sheet. Ordinarily, it is appropriate to design audit procedures that are expected to detect misstatements that aggregate $10,000, $15,000, $20,000 or $30,000 and please explain why.