Answer: 3/5
Work Shown:
Let's say this school has 100 people total. We know that 30 of the people have a cat since 30/100 is the fractional portion of cat owners. Similarly, 18 have both a cat and a dog.
The probability of selecting someone who has both pets, given we know they are a cat owner, is 18/30 = 3/5
To reduce the fraction, divide both parts by the GCF 6.
Note: we don't use 45/100 because we know that whoever selected wasn't solely a dog owner.