Suppose that 23% of the people have a dog, 18% of the people have a cat and 5% of people own both. what is the probability that someone owns a dog or a cat ?

Add units together in order to reach a sum.

In this problem, you are told to find the probability (or %) that someone owns either a dog, OR a cat. If they own both - then they do in fact own a dog or cat. So the formula is:

dog owners % + cat owners % + both owners %

Aka,

The answer is 46% of people own either a cat or a dog, because 23% + 18% + 5% is 46%.