What is the property when u multiply numerator and then multiply denominator and simplify

A new notationWe will start using a new way to indicate simplifying fractions. When a numerator or 
a denominator gets simplified, we will cross it out with a slash and write the new 
numerator or denominator next to it (either above it or below it).The number you divide by (the 4) does not get indicated in any way! You only 
think about it in your mind: “I divide 12 by 4, and get 3. I divide 20 by 4, and get 5.”You may not see any advantage over the “old” method yet, but this shortcut will 
come in handy soon.