If 2 cats have fifteen kittens then there kittens have 20 more kittens how much kittens

If you're saying that those kittens produced 20 kittens each, they would've gotten 300 kittens (which is impossible but oh well). But if you say that they had 20 in all, then you'd count the 20 produced kittens, and the parents of the kittens which makes it 35 kittens in all.

2x15=30x20=600 kittens

In conclusion there are 600 kittens