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