Answer:
The contribution margin for Sam's Bookstore for the first quarter is 0.84 or 84 %
Explanation:
Contribution Margin = Contribution ÷ Sales
Where,
<em>Contribution = Sales - Variable Costs</em>
where,
Sales :
Sales = $ 900,000 
Number of Books Sold = $ 900,000 ÷ $50
                                       = 18,000 books
Variable Costs Calculation :
Cost of goods sold                                                           $630,000
Variable selling expenses ($5 × 18,000 books)               $90,000
Variable administrative expenses( 4% × $ 900,000)       $36,000
Total Variable Costs                                                         $756,000
Therefore,
Contribution Margin =  $756,000÷  $ 900,000
                                   = 0.84 or 84 %