8.4 The distribution of rents of two-bedroom flats in the central district of a city is known to follow a skewed distribution with most flats being relatively cheap to rent but some, in prestige blocks, very expensive. The mean rent of a sample of 48 flats was £403 per month with a standard deviation of £65.

(a) Construct a 90 per cent interval estimate of the mean rent of two-bedroom flats in the area.

(b) The population standard deviation turns out to be £59.

In the light of this new information, determine the size of sample necessary to enable the mean rent to be estimated to within £10, in other words for the error to be £10, with a 90 per cent level of confidence.