In 1877 the price of British-grown wheat averaged 56 shillings and 9 pence a quarter and for the rest of the nineteenth century it never reached within 10 shillings of that figure. In 1878 the price fell to 46 shillings and 5 pence. In 1886 the wheat price decreased to 31 shillings a quarter. By 1885 wheat-growing land declined by a million acres (4,000 km²) (28½%) and the barley area had dwindled greatly also.  Britains dependence on imported grain during the 1830s was 2%; during the 1860s it was 24%; during the 1880s it was 45%, (for wheat alone during the 1880s it was 65%.) The 1881 Census in the United Kingdom showed a decline of 92,250 in agricultural labourers in the ten years since 1871, with an increase of 53,496 urban labourers. Many of these had previously been farm workers who migrated to the cities to find employment, despite agricultural labourers wages being higher than those of Europe. Agricultures contribution to the national income was about 17% in 1871; by 1911 it was less than 7%.
Question: In the forty years after 1871, how many percent of the national income did agriculture's contribution drop?
Answer: 10