In Bombay, for example, the crowding of 10 persons in a room 10 by 15 feet in size is not unusual. Occupancy in tenements ranges from 6 to 9 persons per room, with an overall average of more than 7 persons per room; the bulk of these tenements consist of only one room. In Calcutta, Bombay, Ahmedabad, Cawnpore and Nagpur in India, 60 to 90% of the working-class families and 50% of the middle-class families live in single rooms. Population density reaches 1,200 to 1,500 per acre. Only 10 to 14% of the built-up areas in the bigger cities consist of open spaces in the form of streets. In socialist countries, where construction in cities is mainly directed towards basic equipment to the prejudice of housing, overcrowding of several households in a small flat is quite common. In Japan, the 1978 housing census shows that the average number of persons per room was 0.8 and the average number of rooms per dwelling was 4.5. The corresponding figures for the USA in 1970 were 0.6 and 5.1 respectively, according to the United Nations Statistical Yearbook, 1978.