# Resolved: The practice exam 1, number 6

Hi Victor, thanks for this course.

In the practice exam for combinatorics, question 6, you gave the answer as; Combination with repetition. But the formula you used isn't the right formula for combination with repetition. You used (n+p-1)!/p! instead of (n+p-1)!/(n-1)!p!. Please is there a reason for that? If yes, what reason? Or was it a mistake on your part? Thanks

Hey,

Thank you for the question!

In the explanation of the exercise, Victor has made use of the **'n-choose-k'** notation. It is defined in the following way:

Therefore:

Hope this helps!

Kind regards,

365 Hristina

Thank you, Hristina.

In question 6, the notation has been used to sub in the 'p' and 'n' instead of the formula.

However, I believe only the formula has been taught and not the notation.... This was rather confusing until I looked it up... Might be worth a video or resource explaining this?

Hi Aaron,

thanks for your suggestion! We'll be adding аn explanation to the course notes for this particular formula, as I believe you're right, the notation is somewhat confusing and requires a more thorough explanation. I'll notify you once the new version of the course notes are up.

Best,

365 Eli

Dear all,

The new version of the course notes is now up. We have included the definition of the binomial coefficient on page 10 in the following file:

probability > Section 1 > 1_Course Introduction > Course notes > Probability - Course notes.pdf

Kind regards,

365 Hristina

Thanks both :D

My only note is that it may need to be added to the 'Combinations With Repetition' page too, especially as this is what the question requires

A more involved explanation (than just a mention on page 10) of the notation would be useful. The way it is used in the exam question does not make sense to me so I feel like I'm missing something.

Same issue. I think there is an error with the way the question 6 was solved. Please kindly explain better

Hey Zainab,

Thank you for reaching out.

Could you please elaborate? What is the error you encountered?

Kind regards,

365 Hristina