After the final exam results came out, four students wanted to compare who had done best. Who had the best grades?~(adapted from $$2007$$ Math kangaroo Problems, Level $$1-2$$, Question \#$$4$$)  Alice：$12+8$  Bob：$7+9$  Alan：$14+5$~  Tom：$6+8$  Susan: $6+15$ 
Overseas Competition->Knowledge Point->Calculation Modules->Comparing, Ordering and Estimating->Comparing and Ordering->Comparing and Ordering the Value of Expressions
Alice：$12+8=20$  Bob：$7+9=16$  Alan：$14+5=19$~  Tom：$6+8=14$  Susan: $6+15=21$  $21\textgreater20\textgreater19\textgreater16\textgreater14$ 