Learning Tree, a boutique children's store in Alexandria, sells children's books for $10 per book. As of May 1st, the store had 10 books available for sale; those books were purchased for $2 per book in the prior month. Learning Tree sold 6 books on May 8th and 24 books on May 24th. The store replenished its inventory by purchasing 30 books on May 14th for $3 per book and 8 books on May 28th for $3.50 per book. If Learning Tree uses a periodic inventory tracking system and the FIFO cost flow assumption, 

what is the firm's gross profit from book sales in May?