Context: The share of value added by the cotton textile industry in Britain was 2.6% in 1760, 17% in 1801 and 22.4% in 1831.  Value added by the British woollen industry was 14.1% in 1801.  Cotton factories in Britain numbered approximately 900 in 1797.  In 1760 approximately one-third of cotton cloth manufactured in Britain was exported, rising to two-thirds by 1800.  In 1781 cotton spun amounted to 5.1 million pounds, which increased to 56 million pounds by 1800.  In 1800 less than 0.1% of world cotton cloth was produced on machinery invented in Britain.  In 1788 there were 50,000 spindles in Britain, rising to 7 million over the next 30 years.

Question: How many years did it take for cotton spun to go from 5.1 million pounds to 56 million pounds?

Answer:
19