Ratio of number of book to number of pen sold by a shopkeeper is 25:18, respectively while ratio of number of pen to number of pencil sold by the same shopkeeper is 6:13, respectively. If price of each book, each pen and each pencil is Rs. 40, Rs. 20, and Rs. 10 respectively then revenue generated by shopkeeper is Rs. 21000. Find the number of pen sold by shopkeeper.