Why are 36:8,9:3,72:16 equivalent to 18:4

They all can be divided by the same number like 36:8 divided by 2 is 18:4

u could also set it up like this : 36 18

_ = _

8 4

replace one of the numbers with an X i used 4 now multiply 8 times 18 and divide it by 36 and it will equal 4 which means they are equivalent