The mean temperature of the transformed dataset is $\bar{F} = 77^\circ\text{F}$.

The mean temperature of the deviations from the mean in Fahrenheit is $0^\circ\text{F}$.

If a temperature of $20^\circ\text{C}$ is added to the original dataset, the new mean Celsius temperature will decrease.

If we subtract $25^\circ\text{C}$ from every temperature in the Celsius dataset, the mean of the new dataset is $0^\circ\text{C}$ and the standard deviation is $0^\circ\text{C}$.