A hostel houses 50 residents whose average age is 20 years. The 20 residents on the ground floor have an average age of 17 years. What is the average age of the 30 residents on the upper floors?

All 50 residents total $50\times 20=1000$ years and the ground floor accounts for $20\times 17=340$, so the upper floors total 660 years across 30 people: 660/30 = 22 years. 23 mirrors the 3-year shortfall of the ground floor as a 3-year surplus upstairs, but 20 people short by 3 years is a deficit of 60 years, and 60 shared among 30 people is only 2. 33 divides the correct total of 660 by the wrong group size, 20. 20 simply repeats the hostel average.