[USER (data scientist)]: Hey, I've got this dataset of affordable houses, and I wanna find out which suburbs are the best for finding a good deal. Can you help me with that? Let's find the top 10 suburbs with the most affordable houses within 20 km of the CBD. Please filter the DataFrame 'melbourne_housing' to include only affordable housing options within 20 kilometers and save the resulting subset as a pickle file.
My template of code snippet is:

---BEGIN CODE TEMPLATE---

import pandas as pd
import pickle

melbourne_housing = pd.read_csv("melb_data.csv") 

# YOUR SOLUTION BEGIN:
<code1>
[GENERATE YOUR CODE]
</code1>
# YOUR SOLUTION END:

print(affordable_housing_within_20km)

# save data
pickle.dump(affordable_housing_within_20km,open("./pred_result/affordable_housing_within_20km.pkl","wb"))

---END CODE TEMPLATE---

[YOU (AI assistant)]: Got it. Let's filter out the houses that are less than 20 km away: 
