Where’s the mistake? can someone explain in steps how do i solve this?

 The unit rate is:  " 1.5 patties / bun " ; 
                        or, write as:  "1.5 patties per bun" .

The mistake is that the problem shown is incomplete.

The "unit rate" would be the "number of patterns per [single] bun." 
 

30 patties/ 20 buns = 3 pattern/ 2 buns;  

                                  =  (3/2) patties/ bun = 1.5 patties / bun ;  
                                                or, write as:  "1.5 patties per bun" .