cnloong ,
This is not currently possible with formula questions. Questions (not just formula ones) are independent of each other and you cannot share values between them. This has long been a frustration to statistics teachers (and others) who would like to generate a random set of data and ask for both a mean and a standard deviation (or other similar values).
I have not participated in the Quizzes LTI beta, the documentation does not suggest that anything has changed with that version.
There are a couple of work-arounds that people use, but none of them involve formula questions.
You can generate a multiple-fill-in-the-blanks question. You will have to manually create the question, so it won't be random, but you can make multiple versions of it and put it into a question group to simulate a formula question. Fill-in-the-blank problems are for text and not math, though. They don't understand that 0.120 is the same as .120, .12, or 0.12. You have to allow for every way that someone might write them. The upcoming Quizzes LTI at least allows more options of specifying those: How do I create a Fill in the Blank question in the Quizzes LTI? Also realize that there is no tolerance with fill-in-the-blank questions since they are not considered numeric, they're considered to be text.
You can generate a multiple-drop-down question. You have to specify multiple answers for each, so it removes the potentially desired free-form answers and allows people to pick from a list. You'll have to manually create the question, but as with the multiple-fill-in-the-blank questions, you can put them in a question group to simulate a random question. I use this on some of my quizzes where the questions all relate to each other.
