45% of the children in a school have a dog, 30% have a cat, and 18% have a dog and a cat. What percentage of those who have a cat also have a dog?

P(children have dog) = 45% or 0.45

P(children have cat) = 30% or 0.30

P(children have a dog and a cat) = 18% or 0.18

$P(dog\vert cat)=\frac{P(dog\bigcap cat)}{P(cat)}$

$P(dog\vert cat)=\frac{0.18}{0.30}=0.60$

This means 60% of the children in a school have a cat also have a dog.