Using Python. Transform the letter grades to simplified 4.0 scale. In the simplified scale, there are only 5 available grades, which are `A`, `B`, `C`, `D`, and `F`. Thus, remove the `+` and `-` suffixes from the letter grades before mapping. If the input grade is not valid (e.g., `S`, return np.nan).

When transforming letter grades to 4.0 scale, use the following mapping:

- A => 4.0

- B => 3.0

- C => 2.0

- D => 1.0

- F => 0.0

Resulting data frame should include both letter grades and simplified 4.0 scale grades.

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The solution starts with:

import pandas as pd

grades_df = pd.DataFrame(data = {'John': ['A+'], 'Linus':['B'], 'Alan':['C-'], 'Ada':['D'], 'Grace':['B+'], 'Donald':['F']}, index=['Letter_Grades']).T

...

# The input

# grades_df['4.0scale'] should return

# John 4.0

# Linus 3.0

# Alan 2.0

# Ada 1.0

# Grace 3.0

# Donald 0.0