SAC Prep Variance Question; Answer Doesn't Fit Supplied Formulas?

[Brittank88]

Hi all,

Having some trouble whilst studying for an upcoming probability SAC. The supplied answer/working for a segment of a question asking to calculate variance does not match any formula supplied on the formula sheet or in our work. The formula we're supplied is .

I've attached a photo of the question and official answers, hopefully someone can clear this up for me, and show me how to do these types of questions.

[Bri MT]

The formula they are using is n*p*(1-p), and the reason that they are using this formula rather than the one you have mentioned is because the distribution is binomial

Hope this helps

[Brittank88]

Thanks, you're a lifesaver!

Probability can be so tricky in knowing which formula to apply. p*n*(1-p) is a formula that indeed we had been given in our classwork, however it was obscurely typed out in small at the bottom of some page so I didn't realise it was there as another method. Online searches were returning only the same formula that wasn't correct, so thanks!

[Bri MT]

Thanks, you're a lifesaver!

Probability can be so tricky in knowing which formula to apply. p*n*(1-p) is a formula that indeed we had been given in our classwork, however it was obscurely typed out in small at the bottom of some page so I didn't realise it was there as another method. Online searches were returning only the same formula that wasn't correct, so thanks!

No problem!

It might be worth creating a flowchart in your notes/summary book of "what notation/description is used" to  "what type of distribution is this?" to "how do I find the ___ ?" so that you can refer back to that when you are unsure.

Eg. "there are only two options and no middle ground between them" -> binomial distribution -> use Var = n*p*(1-p)