In 54/100, 2 is the only common factor. Dividing 54 and 100 both by 2, we get 27/50. This is the simplest form of 54/100. Note that 54/100=0.54.
In 90/100, 10 is the only common factor. Divide both 90 and 100 by 10. We get 9/10. That's the simplest form.