## Complicated Grouping

It is sometimes the case that we aren't satisfied with built-in pandas functions, like `.mean()` or `.min()` while grouping.

Look at the column `'Length'`; here, we have the flight length in minutes. Imagine we want to calculate the maximum time in hours for items having the same value in the `'Flight'` column and then in the `'Airline'` one. To do so, we can calculate the maximum value of the column `'Length'` for each group key and then divide it by `60`. Look at the example and the explanation below.

Explanation:

We made the example from the previous chapters a little bit complicated, so with data grouping, everything is the same; let's turn to the `.apply()` function.

• `.apply()` - it helps apply specific function to the needed columns.
• in the `lambda` function, `x` is the argument and `x['Length'].max()/60` is the expression. So, the function finds the maximum value for each group key and divides the aggregated value by `60`.

Your task here is to group data by the airport from which the flight started and then by the weekday. Calculate the minimum amount of time of the sum of the groups' columns `'Length'` and `'Time'` for the groups to figure out how long the flight with delay may take. Follow the algorithm to manage the task:
• Extract the columns `'AirportFrom'`, `'Airline'`, `'Time'`, and `'Length'` from `data` (in this order).
• The order is crucial within the `.groupby()` function; put the columns `'AirportFrom'` and `'Airline'` in this order.
• Calculate the sum of two columns: `'Length'` and `'Time'`. Then find their minimum.