Sampling and Experiments · Superchallenge problem

A town has four neighborhoods with 10, 20, 30 and 40 households, whose average household incomes are $90, $70, $50 and $40 thousand respectively. A surveyor picks ONE neighborhood at random (each equally likely) and then a simple random sample of 5 households within it, and reports the sample mean income. By how many thousand dollars does the expected value of the reported mean exceed the true mean income of all households in the town?