How to multiply a value with the quartile or category it falls in while using pd.cut()?

Question

I have got a dataframe as follows:

import pandas as pd
import numpy as np

data = pd.DataFrame({
    \'a\' : [1, 2, 3, 4, 5],
    \'b\' : [2, 3, 4, 5, 6], 
    \'