def letter$\_$grade$($score:float$)->$ str:

$21$ if score $90\mathrm{.}0$ :

grade $=\text{'}A\text{'}$

elif score $80\mathrm{.}0$ :

grade $=\text{'}B\text{'}$

elif score $70\mathrm{.}0$ :

grade $=\text{'}C\text{'}$

elif score $60\mathrm{.}0$ :

grade $=\text{'}D\text{'}$

else:

grade $=\text{'}F\text{'}$

return grade

$14|$ grade $=78\mathrm{.}50$

$15|$ answer $=$ letter$\_$grade$($grade$)$