feat" problem 3,4,2 added

problem2-595-big-countries.py

@@ -0,0 +1,5 @@
+import pandas as pd
+
+
+def big_countries(world: pd.DataFrame) -> pd.DataFrame:
+    return world[(world['area'] >= 3000000) | (world['population'] >= 25000000)][['name', 'population', 'area']]

problem3-1757-recycle-and-low-fat-products.py

@@ -0,0 +1,5 @@
+import pandas as pd
+
+
+def find_products(products: pd.DataFrame) -> pd.DataFrame:
+    return products[(products['low_fats'] == 'Y') & (products['recyclable'] == 'Y')][['product_id']]

problem4-183-customer-who-never.py

@@ -0,0 +1,8 @@
+import pandas as pd
+
+
+def find_customers(customers: pd.DataFrame, orders: pd.DataFrame) -> pd.DataFrame:
+    df = customers.merge(orders, left_on='id', right_on='customerId', how='left')
+
+    df = df[df['customerId'].isnull()][['name']].rename(columns={'name': 'Customers'})
+    return df