question: How many years did it take for the amount of water boards to be reduced from about 3,500 to 25 in 2011?
#1 return the  amount of water boards was  about 3,500 
#2 return the  amount of water boards was  25 
#3 return year of #1 
#4 return year of #2 
#5 return the  difference of #4 and  #3