Heat Pump COP

An air-to-air heat pump operates between -10°C on the cold side and 45°C on the hot side. What is the max theoretical efficiency?

Hint
For a Carnot heat pump cycle, the Coefficient of Performance (COP) is:
$$$COP_c=\frac{T_H}{(T_H-T_L)}$$$
where $$T_H$$ and $$T_L$$ are the absolute high and low temperatures respectively in either Kelvin or Rankine.
Hint 2
To convert Celsius to Kelvin:
$$$K\:=\:^{\circ}C+273$$$
For a Carnot heat pump cycle, the Coefficient of Performance (COP) is:
$$$COP_c=\frac{T_H}{(T_H-T_L)}$$$
where $$T_H$$ and $$T_L$$ are the absolute high and low temperatures respectively in either Kelvin or Rankine. To convert Celsius to Kelvin:
$$$K=\:^{\circ}C+273$$$
Thus, the high and low temperatures are:
$$$K_H=45^{\circ}C+273=318\:K$$$
$$$K_L=-10^{\circ}C+273=263\:K$$$
$$$COP_c=\frac{318K}{(318K-263K)}=\frac{318}{55}=5.78$$$
5.78