When entering mixed fractions you should be aware that if you put

"10 11/12 + 9 3/8" this is recognized as "10*(11/12) + 9*(3/8)"

So, to get the correct result you should type down

"10+(11/12)+9+(3/8)"

and this way you will get the result 

487/24 = (20*24 + 7)/24 and this is exactly what you ended up with "20 7/24"