Implement mean imputation for missing values.

Calculate the sample mean for Average Heart Rate $($bpm$)$ for the 'Cycling' activity type. Assign the result to avg$\_$hr$\_$cycle.

Filter the df$\_$activities for the 'Cycling' activity type. Create a copy of the result using copy$()$ and assign the copy to df$\_$cycle.

Fill in the missing values for Average Heart Rate $($bpm$)$ in df$\_$cycle with int$($avg$\_$hr$\_$cycle$)$ using the fillna$()$ method.

Count the missing values for all columns in df$\_$run.