How would you find the empirical formula for this... a substance was found by analysis to contain 20% by mass calcium and 80% by mass bromine.
Take a 100 g sample which will give you 80 g bromine and 20 g calcium.
Then convert grams to moles.
moles Ca = 20/atomic mass Ca.
moles Br = 80/atomic mass Br.
Then find the ratio of one element to the other. Remember you want small whole numbers with no number lower than 1.
Post your work if you get stuck.