Ответ на задание 1.

import pandas as pd

df=pd.DataFrame({'age':[18,25,40], 'income':[100,180,350], 'buys':[0,1,1]})

print(df[['age','income']], df['buys'])

Ответ на задание 2.

from sklearn.model_selection import train_test_split

X=list(range(100)); a,b=train_test_split(X,test_size=0.2,random_state=42)

print(len(a),len(b))

Ответ на задание 3.

from sklearn.datasets import load_iris

from sklearn.model_selection import train_test_split

from sklearn.neighbors import KNeighborsClassifier

X,y=load_iris(return_X_y=True)

a,b,c,d=train_test_split(X,y,random_state=42,stratify=y)

print(KNeighborsClassifier(n_neighbors=5).fit(a,c).score(b,d))