จำนวนงานโดยเฉลี่ยของ Snowdon Ranger กับผู้สร้าง Vulcan Foundry คือเท่าใด
What is the average work number of Snowdon Ranger with the builder Vulcan Foundry?
df = pd.DataFrame(columns=['works_number', 'builder', 'name'])
df[(df['builder'] == 'vulcan foundry') & (df['name'] == 'snowdon ranger')]['works_number'].mean()