คลาสใดมีปริมาณน้อยกว่า 3 แบบ 4-6-0
Which class has less than 3 in quantity with  a type of 4-6-0?
df = pd.DataFrame(columns=['class','type','quantity'])
df[(df['type'] == '4-6-0') & (df['quantity'] < 3)]['class']