Calculating P1 for the entropy

Hi Everyone, I have a question regarding the P1 calculation for the entropy.

In the image below, P1 for the left branch is calculated as 4/5. How did we confirm/know that there are 4 images of cats out of the 5 even before we classified them?

Thanks for your time, Manjunath.

The data set was labeled by a cat expert before it was presented to you.