The combining ratio of potassium (K) and bromine (Br) is 1:1. This means that one atom of potassium combines with one atom of bromine to form one molecule of potassium bromide (KBr).

Here's why:

* Potassium is in group 1 of the periodic table and has a +1 charge when it becomes an ion.

* Bromine is in group 17 of the periodic table and has a -1 charge when it becomes an ion.

To form a neutral compound, the charges must balance out. Therefore, one potassium ion (K+) combines with one bromide ion (Br-) to form potassium bromide (KBr).