AMC12 2005 12A, Problem 5

The average (mean) of 20 numbers is 30, and the average of 30 other numbers is 20. What is the average of all 50 numbers?

Solution: This is just the harmonic mean. Answer is $\frac{2*20*30}{20+30}=24 \ $. Solution by franzliszt $\fbox{24}$.

Answer: 24