The average of five numbers is 20. One of the five is taken away, and the average of the four that remain is 18. Which number was taken away?

Turn each average back into a total. Five numbers averaging 20 add up to $5\times 20=100$. The four that remain average 18, so they add up to $4\times 18=72$. The number taken away is the difference: 100 − 72 = 28. The traps: 2 is the drop in the average, not the number removed — removing a number that is bigger than the average pulls the average down by only a little; 18 and 20 simply repeat the new and the old averages.