In the 17th century there were many of these independent local bodies levying their own taxes and administering justice. This early form of local government played a role in the development of a political system in the Netherlands that was decentralised and dependent on communal cooperation. Widespread experience with decentralized government was a factor in the formation of the Dutch Republic in the 16th and 17th centuries. The mandate of Rijkswaterstaat , established in 1798 under French rule, was to centralise water control in the Netherlands. Local water boards refused to give up their autonomy however, so Rijkswaterstaat ended up working alongside the local water boards. Today Rijkswaterstaat has responsibility for major water control structures and other infrastructure like motorways. By 1850 there were about 3,500 water boards in the country. In modern times water boards merged as they dealt with joint  interests. Mergers eventually reduced the number to 25 water boards in 2011. The tasks of water boards remain basically unchanged. Having a rich history dating back to the Middle Ages, they are the oldest governing bodies and the oldest democratic institutions in the Netherlands. Dutch water boards have their own coat of arms, a colourful reminder of their importance in Dutch history. The historic buildings that used to house the water boards are another legacy. Called gemeenlandshuis or waterschaphuis, these charming old buildings can be found at the heart of many Dutch towns.
Question: How many years did it take for the amount of water boards to be reduced from about 3,500 to 25 in 2011?
Answer: 161