An income statement for Joshua Sloan LLC for the first quarter of the year is presented below: Joshua Sloan LLC Income Statement For Quarter Ended March 31 900,000 Sales Cost of goods sold Gross margin Selling and administrative expenses Selling 630,000 270,000 100,000 204,000 104,000 Administration 66,000 Net operating income On average, a book sells for $50. Variable selling expenses are $5 per book with the remaining selling expenses being fixed. The variable administrative expenses are 5% of sales with the remainder being fixed. Determine the period cost formula with "X" representing the of books sold