What to do if I find actual exam marks higher than the marks I made for my exam? In course specification for the course I teach there should be two mid-term exams. For each one that total marks should be 20.  I forgot about that and gave the students the first mid-term with only 15 multiple choice (1 mark for each). 

Now how can I correct my mistake? Should I give them a bonus of 5 marks or what is the best way to correct my mistake?

It also would be helpful to suggest a book that discuss such situations.
Options: 
Re-scale the grades by multiplying a student's score, x, by 20/15. Thus a score of 12 out of 15 (which is 80%) would become 16 out of 20 (still 80%).

Just give them five more questions. Did you plan to get more data for grading purposes? You can always get more.