Conversion scheme $($from score to letter$)$:

$100-93$: A

$92-90$: A$-$

$89-87$: $B^{+}$

$86-83$: B

$82-80$ : B$-$

$79-77$: C$+$

$76-70$: C

$69-60$: D

$59-0$:$F$