Contemporary documents show that the British continued the physical occupation of Chumbi Valley until February 8, 1908, after having received the full payment from China. In early 1910, Qing China sent a military expedition of its own to Tibet for direct rule. However, the Qing dynasty was overthrown in the Xinhai Revolution, which began in October 1911. Although the Chinese forces departed once more in 1913, the First World War and the Russian Revolution isolated the now independent Tibet, reducing Western influence and interest.  Ineffectual regents ruled during the 14th Dalai Lama's infancy and China began to reassert its control, a process that culminated in 1950-1951 with the Chinese invasion of Tibet by a newly-formed Communist China. The position of British Trade Agent at Gyangzê was occupied from 1904 until 1944. It was not until 1937, with the creation of the position of "Head of British Mission Lhasa", that a British officer had a permanent posting in Lhasa itself. The British seem to have misread the military and diplomatic situation, for the Russians did not have the designs on India that the British imagined, and the campaign was politically redundant before it began. Russian arms in Tibet amounted to no more than thirty Russian government rifles, and the whole narrative of Russian influence, and the Czar's ambitions, was dropped. The defeats the Russians experienced in the Russo-Japanese war that began in February 1904 further altered perceptions of the balance of power in Asia, and the Russian threat. However, it has been argued that the campaign had "a profound effect upon Tibet, changing it forever, and for the worse at that, doing much to contribute to Tibet's loss of innocence."
Question: How many years after the British left the Chumbi Valley was it before the Communist Chinese invaded Tibet?
Answer: 42