hi Treecko. your grades for B+ & B overlaps. 77-79 & 73-79. assuming the latter is 73-76, place the max number to get the particular grade say in I6:I17.
100
89
84
79
77
72
69
66
62
59
56
52

then try this in C6:
=INDEX(G$6:G$17,MATCH($B6,$I$6:$I$17,-1))

copy over. if you don't want the values in Column I, then:
=INDEX(G$6:G$17,MATCH($B6,{100;89;84;79;77;72;69;66;62;59;56;52},-1))