Smart Engineers

A boy goes to shop and buys a big book.

He takes the book to a medical student and asks him how much time he will take to finish the book.

The Medical guy says 6 month's minimum   

Then he goes to a law student and asks the same question, he says minimum 3 months.

Then he goes to an engineer and asks the same question how much time will you take to finish the book???

The engineer says. "When is the exam?"