Buying Books with father&sons

Two friends, Arun and Boby, go to a bookshop, together with their sons Praveen and Tejas. 

• All four of them buy some books; each book costs a whole amount in Rupees. 
• When they leave the bookshop, they notice that both fathers have spent 21rs/- more than    their respective sons have.
• Moreover, each of them paid per book the same amount  as books that he bought. 
• The difference between the number of books of Arun and Praveen is five.

    Who is the father of Tejas?

Watch this space for answer in a week.