data['kitchen_k'] = data['kitchen_area'] / data['total_area']
kitchen_mean_k = data.groupby('rooms').agg({'kitchen_k' : 'mean'})


for room in kitchen_mean_k.index:
    data.loc[(data['rooms'] == room) & (data['kitchen_area'].isnull()), 'kitchen_area'] = data.loc[(data['rooms'] == room) & (data['kitchen_area'].isnull()), 'total_area'] * kitchen_mean_k.loc[room, 'kitchen_k']