25 is our base, so, we need to find out what 19 out of 25 is what percentage. Let's just start with something a little small, like 30%
(We can experiment with different percentages using our trusty calculator)
30% of 25 = 7.5
We are way off, we need to bring it a bit higher if it is to reach 19. Let's try 67%
67% of 25 = 16.75
Not quite but close, lets try something a bit higher, like 76%.
76% of 25 = 19
So, 19 out of 25 is 76% all we need to do now is subtract that from 100 to get the remaining percentage.
100 - 76 = 24
24% of the students don't have a pet. :D