The most important existing communal shelters were the London Underground stations. Although many civilians had used them as such during the First World War, the government in 1939 refused to allow the stations to be used as shelters so as not to interfere with commuter and troop travel, and the fears that occupants might refuse to leave. Underground officials were ordered to lock station entrances during raids; but by the second week of heavy bombing the government relented and ordered the stations to be opened. Each day orderly lines of people queued until 4 pm, when they were allowed to enter the stations. In mid-September 1940 about 150,000 a night slept in the Underground, although by the winter and spring months the numbers had declined to 100,000 or less. Noises of battle were muffled and sleep was easier in the deepest stations, but many were killed from direct hits on several stations.

Answer this question, if possible (if impossible, reply "unanswerable"): In 1940 how many people were sleeping in the Underground stations?
150,000 a night